Tuesday, March 31, 2020

Age, Sex, Existing Conditions of COVID-19 Cases and Deaths

There are two sources that provide age, sex, and comorbidity statistics:

The Report of the WHO-China Joint Mission published on Feb. 28 by WHO, [2] which is based on 55,924 laboratory confirmed cases. The report notes that "The Joint Mission acknowledges the known challenges and biases of reporting crude CFR early in an epidemic" (
A paper by the Chinese CCDC released on Feb. 17, which is based on 72,314 confirmed, suspected, and asymptomatic cases of COVID-19 in China as of Feb. 11, and was published in the Chinese Journal of Epidemiology

We will list data from both, labeling them as "confirmed cases" and "all cases" respectively in the tables.

Age of Coronavirus Deaths

COVID-19 Fatality Rate by AGE:

*Death Rate = (number of deaths / number of cases) = probability of dying if infected by the virus (%). This probability differs depending on the age group. The percentages shown below do not have to add up to 100%, as they do NOT represent share of deaths by age group. Rather, it represents, for a person in a given age group, the risk of dying if infected with COVID-19.

AGE	DEATH RATE confirmed cases	DEATH RATE all cases
80+ years old	21.9%	14.8%
70-79 years old		8.0%
60-69 years old		3.6%
50-59 years old		1.3%
40-49 years old		0.4%
30-39 years old		0.2%
20-29 years old		0.2%
10-19 years old		0.2%
0-9 years old		no fatalities

*Death Rate = (number of deaths / number of cases) = probability of dying if infected by the virus (%). The percentages do not have to add up to 100%, as they do NOT represent share of deaths by age group.

In general, relatively few cases are seen among children.

Sex ratio

COVID-19 Fatality Rate by SEX:

*Death Rate = (number of deaths / number of cases) = probability of dying if infected by the virus (%). This probability differs depending on sex. When reading these numbers, it must be taken into account that smoking in China is much more prevalent among males. Smoking increases the risks of respiratory complications.

SEX	DEATH RATE confirmed cases	DEATH RATE all cases
Male	4.7%	2.8%
Female	2.8%	1.7%

*Death Rate = (number of deaths / number of cases) = probability of dying if infected by the virus (%). The percentages do not have to add up to 100%, as they do NOT represent share of deaths by sex.

Pre-existing medical conditions (comorbidities)

Patients who reported no pre-existing ("comorbid") medical conditions had a case fatality rate of 0.9%. Pre-existing illnesses that put patients at higher risk of dying from a COVID-19 infection are:

COVID-19 Fatality Rate by COMORBIDITY:

*Death Rate = (number of deaths / number of cases) = probability of dying if infected by the virus (%). This probability differs depending on pre-existing condition. The percentage shown below does NOT represent in any way the share of deaths by pre-existing condition. Rather, it represents, for a patient with a given pre-existing condition, the risk of dying if infected by COVID-19.

PRE-EXISTING CONDITION	DEATH RATE confirmed cases	DEATH RATE all cases
Cardiovascular disease	13.2%	10.5%
Diabetes	9.2%	7.3%
Chronic respiratory disease	8.0%	6.3%
Hypertension	8.4%	6.0%
Cancer	7.6%	5.6%
no pre-existing conditions		0.9%

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Coronavirus (COVID-19) Mortality Rate

3.4% Mortality Rate estimate by the World Health Organization (WHO) as of March 3

In his opening remarks at the March 3 media briefing on Covid-19, WHO Director-General Dr Tedros Adhanom Ghebreyesus stated:

“Globally, about 3.4% of reported COVID-19 cases have died. By comparison, seasonal flu generally kills far fewer than 1% of those infected.

Initial estimate was 2%

Initially, the World Health Organization (WHO) had mentioned 2% as a mortality rate estimate in a press conference on Wednesday, January 29 [1][2] and again on February 10. However, on January 29 WHO specified that this was a very early and provisional estimate that might have changed. Surveillance was increasing, within China but also globally, but at the time it was said that:

We don't know how many were infected ("When you look at how many people have died, you need to look at how many people where infected, and right now we don't know that number. So it is early to put a percentage on that.
The only number currently known is how many people have died out of those who have been reported to the WHO.
It is therefore very early to make any conclusive statements about what the overall mortality rate will be for the novel coronavirus, according to the World Health Organization

Mortality Rate as of Feb. 20 in China (findings from the Report of the WHO-China Joint Mission)

The Report of the WHO-China Joint Mission published on Feb. 28 by WHO is based on 55,924 laboratory confirmed cases. The report notes that "The Joint Mission acknowledges the known challenges and biases of reporting crude CFR early in an epidemic" . Here are its findings on Case Fatality Ratio, or CFR (the mortality rate):

"As of 20 February, 2,114 of the 55,924 laboratory confirmed cases have died (crude fatality ratio [CFR: 3.8%) (note: at least some of whom were identified using a case definition that included pulmonary disease).

The overall CFR varies by location and intensity of transmission (i.e. 5.8% in Wuhan vs. 0.7% in other areas in China).

In China, the overall CFR was higher in the early stages of the outbreak (17.3% for cases with symptom onset from 1-10 January) and has reduced over time to 0.7% for patients with symptom onset after 1 February. "

The Joint Mission noted that the standard of care has evolved over the course of the outbreak.

Mortality Rate, as discussed by the National Health Commission (NHC) of China on Feb. 4

Asked at a press conference on February 4 what the current mortality rate (or case fatality rate, CFR) is, an official with China NHC said that

The formula they are using is: cumulative current total deaths / current confirmed cases. Therefore, as of 24:00 on Feb. 3, the formula used was 425/20,438.
Based on this figure, the national mortality rate to date was 2.1% of confirmed cases.
There might be mild cases and other cases not reported.
97% of the country's total deaths (414) were in the Hubei Province.
Mortality rate in Wuhan was 4.9%.
Mortality rate in the Hubei Province was 3.1%.
Mortality rate nationwide was 2.1%.
Fatality rate in other provinces was 0.16%.
Deaths in Wuhan were 313, accounting for 74% of China's total.
Most of the cases were still mild cases, therefore there was no need to panic.
Asked why Wuhan was so much higher than the national level, the NHC official replied that it was for lack of resources, citing as an example that there were only 110 critical care beds in the three designated hospitals where most of the cases were sent.
National mortality rate was basically stable, as of Feb. 4 at 2.1%, and it was 2.3% at the beginning of the epidemic, which can be seen as a slight decline.
Front the analysis of death cases, it emerged that the demographic profile was mainly male, accounting for 2/3, females accounting for 1/3, and is mainly elderly, more than 80% are elderly over 60 years old, and more than 75% had underlying diseases present such as cardiovascular and cardiovascular diseases, diabetes and, in some cases, tumor.
Elderly people with basic diseases, as long as they have pneumonia, were clinically a high-risk factor regardless of whether it is a coronavirus or not, and the case fatality rate was also very high, so it is not that the case fatality rate of pneumonia is high because of the infection with the new coronavirus. "This point must be explained to everyone," concluded the NHC official.[7]

Preliminary study providing a tentative 3% estimate for case fatality rate

A preliminary study published on The Lancet on January 24 provided an early estimation of 3% for the overall case fatality rate. Below we show an extract (highlights added for the relevant data and observations):

Of the 41 patients in this cohort, 22 (55%) developed severe dyspnoea and 13 (32%) required admission to an intensive care unit, and six died.

Hence, the case-fatality proportion in this cohort is approximately 14.6%, and the overall case fatality proportion appears to be closer to 3%.

However, both of these estimates should be treated with great caution because not all patients have concluded their illness (ie, recovered or died) and the true number of infections and full disease spectrum are unknown.

Importantly, in emerging viral infection outbreaks the case-fatality ratio is often overestimated in the early stages because case detection is highly biased towards the more severe cases.

As further data on the spectrum of mild or asymptomatic infection becomes available, one case of which was documented by Chan and colleagues, the case-fatality ratio is likely to decrease.

Nevertheless, the 1918 influenza pandemic is estimated to have had a case-fatality ratio of less than 5% but had an enormous impact due to widespread transmission, so there is no room for complacency.

Fatality rate can also change as a virus can mutate, according to epidemiologists.

Death rate among patients admitted to hospital

A study on 138 hospitalized patients with 2019-nCoV infection, published on February 7 on JAMA, found that 26% of patients required admission to the intensive care unit (ICU) and 4.3% died, but a number of patients were still hospitalized at the time. A previous study had found that, out of 41 admitted hospital patients, 13 (32%) patients were admitted to an ICU and six (15%) died.

Days from first symptom to death

The Wang et al. February 7 study published on JAMA found that the median time from first symptom to dyspnea was 5.0 days, to hospital admission was 7.0 days, and to ARDS was 8.0 days.

Previously. the China National Health Commission reported the details of the first 17 deaths up to 24 pm 22 Jan 2020. A study of these cases found that the median days from first symptom to death were 14 (range 6-41) days, and tended to be shorter among people of 70 year old or above (11.5 [range 6-19] days) than those with ages below 70 year old (20 [range 10-41] days.

Median Hospital Stay

The JANA study found that, among those discharged alive, the median hospital stay was 10 days.

Comparison with other viruses

For comparison, the case fatality rate with seasonal flu in the United States is less than 0.1% (1 death per every 1,000 cases).

Mortality rate for SARS was 10%, and for MERS 34%.

Virus	Death Rate
Wuhan Novel Coronavirus (2019-nCoV)	2%*
SARS	9.6%
MERS	34%
Swine Flu	0.02%

*estimate

How to calculate the mortality rate during an outbreak

At present, it is tempting to estimate the case fatality rate by dividing the number of known deaths by the number of confirmed cases. The resulting number, however, does not represent the true case fatality rate and might be off by orders of magnitude [...]

A precise estimate of the case fatality rate is therefore impossible at present.

2019-Novel Coronavirus (2019-nCoV): estimating the case fatality rate – a word of caution - Battegay Manue et al., Swiss Med Wkly, February 7, 2020

The case fatality rate (CFR) represents the proportion of cases who eventually die from a disease.

Once an epidemic has ended, it is calculated with the formula: deaths / cases.

But while an epidemic is still ongoing, as it is the case with the current novel coronavirus outbreak, this formula is, at the very least, "naïve" and can be "misleading if, at the time of analysis, the outcome is unknown for a non negligible proportion of patients." [8]

(Methods for Estimating the Case Fatality Ratio for a Novel, Emerging Infectious Disease - Ghani et al, American Journal of Epidemiology).

In other words, current deaths belong to a total case figure of the past, not to the current case figure in which the outcome (recovery or death) of a proportion (the most recent cases) hasn't yet been determined.

The correct formula, therefore, would appear to be:

CFR = deaths at day.x / cases at day.x-{T}(where T = average time period from case confirmation to death)

This would constitute a fair attempt to use values for cases and deaths belonging to the same group of patients.

One issue can be that of determining whether there is enough data to estimate T with any precision, but it is certainly not T = 0 (what is implicitly used when applying the formula current deaths / current cases to determine CFR during an ongoing outbreak).

Let's take, for example, the data at the end of February 8, 2020: 813 deaths (cumulative total) and 37,552 cases (cumulative total) worldwide.

If we use the formula (deaths / cases) we get:

813 / 37,552 = 2.2% CFR (flawed formula).

With a conservative estimate of T = 7 days as the average period from case confirmation to death, we would correct the above formula by using February 1 cumulative cases, which were 14,381, in the denominator:

Feb. 8 deaths / Feb. 1 cases = 813 / 14,381 = 5.7% CFR (correct formula, and estimating T=7).

T could be estimated by simply looking at the value of (current total deaths + current total recovered) and pair it with a case total in the past that has the same value. For the above formula, the matching dates would be January 26/27, providing an estimate for T of 12 to 13 days. This method of estimating T uses the same logic of the following method, and therefore will yield the same result.

An alternative method, which has the advantage of not having to estimate a variable, and that is mentioned in the American Journal of Epidemiology study cited previously as a simple method that nevertheless could work reasonably well if the hazards of death and recovery at any time t measured from admission to the hospital, conditional on an event occurring at time t, are proportional, would be to use the formula:

CFR = deaths / (deaths + recovered)

which, with the latest data available, would be equal to:

37,877 / (37,877 + 166,411) = 19% CFR (worldwide)

If we now exclude cases in mainland China, using current data on deaths and recovered cases, we get:

34,572 / (34,572 + 90,359) = 27.7% CFR (outside of mainland China)

The sample size above is limited, and the data could be inaccurate (for example, the number of recoveries in countries outside of China could be lagging in our collection of data from numerous sources, whereas the number of cases and deaths is more readily available and therefore generally more up to par).

There was a discrepancy in mortality rates (with a much higher mortality rate in China) which however is not being confirmed as the sample of cases outside of China is growing in size. On the contrary, it is now higher outside of China than within.

That initial discrepancy was generally explained with a higher case detection rate outside of China especially with respect to Wuhan, where priority had to be initially placed on severe and critical cases, given the ongoing emergency.

Unreported cases would have the effect of decreasing the denominator and inflating the CFR above its real value. For example, assuming 10,000 total unreported cases in Wuhan and adding them back to the formula, we would get a CFR of 17.7% (quite different from the CFR of 19% based strictly on confirmed cases).

Neil Ferguson, a public health expert at Imperial College in the UK, said his “best guess” was that there were 100,000 affected by the virus even though there were only 2,000 confirmed cases at the time. [

Without going that far, the possibility of a non negligible number of unreported cases in the initial stages of the crisis should be taken into account when trying to calculate the case fatally rate.

As the days go by and the city organized its efforts and built the infrastructure, the ability to detect and confirm cases improved. As of February 3, for example, the novel coronavirus nucleic acid testing capability of Wuhan had increased to 4,196 samples per day from an initial 200 samples.

A significant discrepancy in case mortality rate can also be observed when comparing mortality rates as calculated and reported by China NHC: a CFR of 3.1% in the Hubei province (where Wuhan, with the vast majority of deaths is situated), and a CFR of 0.16% in other provinces (19 times less).

Finally, we shall remember that while the 2003 SARS epidemic was still ongoing, the World Health Organization (WHO) reported a fatality rate of 4% (or as low as 3%), whereas the final case fatality rate ended up being 9.6%.

Comparison Novel Coronavirus (COVID-19) with other viruses

Comparisonwith other viruses

For comparison, the incubation period for the common flu (seasonal influenza) is typically around 2 days. Incubation period for other coronaviruses: SARS 2-7 days; MERS 5 days typically (range 2-14 days).

Virus	Incubation Period (typical cases)
Novel Coronavirus (COVID-19)	2-14 or 0-24 days *
SARS	2-7 days, as long as 10 days
MERS	5 days (range: 2-14)
Swine Flu	1-4 days, as long as 7 days
Seasonal Flu	2 days (1-4 range)

Coronavirus Incubation Period

Coronavirus Incubation Period:

2 - 14 days

Possible outliers: 0 - 27 days

Summary of findings:

2-14 days represents the current official estimated range for the novel coronavirus COVID-19.

COVID-19 Incubation Period

The incubation period (time from exposure to the development of symptoms) of the virus is estimated to be between 2 and 14 days based on the following sources:

The World Health Organization (WHO) reported an incubation period for COVID-19 between 2 and 10 days. [1]
China’s National Health Commission (NHC) had initially estimated an incubation period from 10 to 14 days [2].
The United States' CDC estimates the incubation period for COVID-19 to be between 2 and 14 days [3].
DXY.cn, a leading Chinese online community for physicians and health care professionals, is reporting an incubation period of "3 to 7 days, up to 14 days".

The estimated range will be most likely narrowed down as more data becomes available.

Incubation period of up to 24 days?

The incubation period has been found to be as long as 24 days (range: 0-24 days; median: 3.0 days) in a study published on February 9.

The WHO said in a press conference on February 10 that:

a very long incubation period could reflect a double exposure.
24 days represented an outlier observation that must be taken into consideration in the context of the main finding of the study.
WHO is not considering changing recommendations regarding incubation periods.

More recently, however, a case with an incubation period of 19 days was observed in a JAMA study published on Feb. 21., and another case with an incubation period of 27 days was reported by Hubei Province on Feb. 22

Incubation period of 5.2 days on average

A Chinese study published in the New England Journal of Medicine on Jan. 30[7], has found the incubation period to be 5.2 days on average, but it varies greatly among patients. The Chinese team conducting the study said their findings support a 14-day medical observation period for people exposed to the pathogen.

Below is an extract of the study findings (highlight added by Worldometer):

Among the first 425 patients with confirmed NCIP, the median age was 59 years and 56% were male. The majority of cases (55%) with onset before January 1, 2020, were linked to the Huanan Seafood Wholesale Market, as compared with 8.6% of the subsequent cases.

The mean incubation period was 5.2 days (95% confidence interval [CI], 4.1 to 7.0), with the 95th percentile of the distribution at 12.5 days.

In its early stages, the epidemic doubled in size every 7.4 days. With a mean serial interval of 7.5 days (95% CI, 5.3 to 19), the basic reproductive number was estimated to be 2.2 (95% CI, 1.4 to 3.9).

Conclusions On the basis of this information, there is evidence that human-to-human transmission has occurred among close contacts since the middle of December 2019. Considerable efforts to reduce transmission will be required to control outbreaks if similar dynamics apply elsewhere. Measures to prevent or reduce transmission should be implemented in populations at risk.

Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia - Qun Li et al., New England Journal of Medicine, Jan. 29, 2020

Incubation Period in Travelers from Wuhan

A study financed by the Netherlands Ministry of Health and published on Eurosurveillance,[10] analyzed data on 88 cases with known travel history (to and) from Wuhan which were detected between 20 and 28 January as being infected with COVID-19.

The mean incubation period was estimated to be 6.4 days. The incubation period ranges from 2.1 to 11.1 days. The upper limit of 11.1 days could be considered conservative.[10]

The importance of knowing the incubation period

Understanding the incubation period is very important for health authorities as it allows them to introduce more effective quarantine systems for people suspected of carrying the virus, as a way of controlling and hopefully preventing the spread of the virus.

Coronavirus Symptoms (COVID-19)

Last updated: February 29, 4:40 GMT - We will continue to update and improve this page as we gather new information and details.

Reported illnesses have ranged from people with mild symptoms to people being severely ill and dying.

Symptoms can include:

Fever

Cough

Shortness of breath

Typical Symptoms

COVID-19 typically causes flu-like symptoms including a fever and cough.

In some patients - particularly the elderly and others with other chronic health conditions - these symptoms can develop into pneumonia, with chest tightness, chest pain, and shortness of breath.

It seems to start with a fever, followed by a dry cough.

After a week, it can lead to shortness of breath, with about 20% of patients requiring hospital treatment.

Notably, the COVID-19 infection rarely seems to cause a runny nose, sneezing, or sore throat (these symptoms have been observed in only about 5% of patients). Sore throat, sneezing, and stuffy nose are most often signs of a cold.

How long do symptoms last?

Using available preliminary data, the Report of the WHO-China Joint Mission published on Feb. 28 by WHO, [5] which is based on 55,924 laboratory confirmed cases, observed the following median time from symptoms onset to clinical recovery:

mild cases: approximately 2 weeks
severe or critical disease: 3 - 6 weeks
time from onset to the development of severe disease (including hypoxia): 1 week

Among patients who have died, the time from symptom onset to outcome ranges from 2 - 8 weeks.

Symptoms observed in hospitalized patients with COVID-19

Below we list the symptoms, with percentages representing the proportion of patients displaying that symptom, as observed in hospitalized patients tested and identified as having laboratory-confirmed COVID-19 infection. These findings refer to hospitalized patients, therefore generally representing serious or critical cases. The majority of cases of COVID-19 (about 80%) is mild.

ELECTRICAL SYSTEM Questions & Answers

ELECTRICAL SYSTEM

• What are the design objectives of Electrical System?

a. To evacuate generated electrical power.
b. To provide required power to SUT, UT, DG, UPS, and CUPS.
c. To provide required emergency power from onsite DG, UPS & CUPS.
d. To provide Fast transfer in event of Class IV failure. Emergency transfer in
events of Class III and Class II failure.
e. Load shedding in event of one DG available.
f. To provide un-interruptible or few milli seconds interrupted power supply by
UPS and un-interruptible power supply by CUPS.
g. To provide operational flexibility by providing required qualities of requirement.
h. To provide isolation, Alarms, indication, protection of the system.
i. To provide fire protection.
j. To provide surge and lightning protection.
k. To provide adequate lighting.
l. To provide equipment earthing, system earthing, and personnel protection.
m. To provide necessary electrical and physical isolation of electrical equipments.

• What are the design guidelines for electrical system?

a. All safety related equipments are in control building, SRPH and are designed for
SSE conditions. As per studies seismic condition is not there within 5 kms and
nearest zone is away from 20 kms.
b. Safety related equipments are separated from suitable fire barriers of 3 hrs rating
by horizontal and vertical clearances and from turbine building which are houses
high energy rotating equipments and where potential for fire is exist.
c. Separate switchyard control is provided in case of non-availability of main
control room with line and bus coupler protection and bus bar protections.
Control room posses SUT, UT, GT, Generator and all classes of power supply
control and protections.
d. Protection panels of Generator, GT, and UT are separated from SUT in physical
to have system flexibility.
e. SCADA is provided in CER, TB and in switchyard separately.
f. EMTR for each A and B groups are separated.
g. Control supply for switchyard is separated from operating island.
h. To reduce fault level in lighting circuits separate 280-kVA transformer is
provided.

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Predictive (preventive) maintenance on Induction Motor. Questions & Answers

Predictive (preventive) maintenance on Induction Motor.

1. What are the reasons for high current in motor?

a. High frequency – at 51 Hz current will be 105% of the normal current.
b. Low frequency – at 47.8 Hz current will be 102% of the normal current.
c. High voltage.
d. Under voltage.
e. Mechanical over loading.
f. Process requirement.

2. What are the reasons for unbalanced current in motor?

a. Loose power cable connection.
b. Voltage unbalance.
c. Short-circuited turns of coils of winding.

3. What are the reasons for vibration in the motor?

Vibration could be because of mechanical faults and electrical faults.
1. Mechanical faults.
a. Wrong alignment of the motor on foundation.
b. Wrong installation.
c. Improper fitting of bearing and cooling fans.
d. Periodic impulse loads such as reciprocating compressors.
e. Pulley of heavy weight which cause bending of motor shaft resulting in non
uniform air gap.
f. Damage of bearing or bad bearing.
g. Bad coupling.
h. If the axial alignment of the motor and the driven machine is incorrect and
rotor is allowed to contact its axial stops, high axial vibrations may occur,
together with high bearing temperature high and even bearing failure.
i. Machine base and foundation problem.
j. Under sized bearing.

2. Electrical faults.

a. Air gas dissymetry.
b. Broken rotor bars.
c. Slackened stator core.
d. Slackened rotor core.
e. Interturn short in the rotor winding in the two-pole machine.
f. Unbalance in rotor winding.
g. Unbalance power supply voltages.
If the vibration is because of electrical fault, de-energise the machine and
watch the vibration as it runs down.

The possible vibration frequencies observed are
a. Twice the power supply frequency – it indicates that the vibration is developed
by unbalanced power supply voltages, unbalanced air gap, unbalance in rotor
winding, slackened stator core etc.
b. Multiple of power frequency – the stator and rotor slots co-ordinate to develop
radial lines of force to deform and pulsate the cores.
c. Twice the slip frequency – magnetic unbalance due to unbalance air gaps,
slackened rotor core, interturn short in the rotor-winding etc. of two-pole
machine.
d. Beat (Humming) – in case of two-pole machine the beat is developed when
the vibration of twice as much as power frequency developed between the
stator and rotor is superimposed on the vibration of twice the slip frequency
developed due to irregular air gap.

4. What are the reasons for winding temperature high in the motor?

For motors having class – B insulation the temperature should not be more than
110°C and for motors having class – F insulation the temperature should not be more
than 130°C. In case temperature is more, then the following could be the possible
reasons.
1. Electrical overloads.
a. Over and under voltage.
b. Over and under frequency.
c. Voltage unbalance. Voltage unbalance create unbalance of currents and
increase in temperature which will be 2*(% voltage unbalance)* (% voltage
unbalance)*.
(% Voltage unbalance) = 100 * maximum deviation from average voltage
average voltage.
For instance if voltages are 390V, 410V & 440V,
% Voltage variation = 100*(440-390+410+440) (440-390+410+440) = 6.45%.
3 3
Therefore increase in temperature rise = 2*(6.45)*(6.45) = 83°C (approximately).
d. Voltage transients and interruptions.
e. Loose connection at motor terminals.
f. Unbalance current.
g. Single phasing (if OLR protection is not working).
h. Long acceleration cycle.
i. Unusual system grounding conditions.

2. Mechanical overloads.

a. Locked rotor.
b. Heavy starting.
c. Bearing problem.
d. Overload in continuous duty and intermittent duty.
3. Environmental overloads.
a. Excessive temperature of cooling medium or ambient temperature.
b. Restricted flow of cooling.
c. Reduction in the density of cooling medium.
d. Heat transfer from machine parts connected to the motor.
4. Others.
a. Excessive number of switching operations.

5. What are the reasons for bearing temperature high?

Temperature of bearing should not be more than 90°C. In case temperature is higher
than the 90°C the following could be the possible reasons.
a. Inadequate lubricants inside the bearing.
b. Faulty bearing.
c. Bearing is jammed.
d. Over greasing.
e. Improper grade of lubricant.

6. What are the reasons for abnormal sound or noise?

Motors in general should run very quietly and no abnormal noise is desired.
However if noise is there, it could be because of following reasons.
a. Windage noise – the noise due to ventilating system, (whistling noise).
b. Bearing noise – the noise due to its rolling contact.
c. Unusual noise – some defects inside the motor (example – motor bar failure).
d. Deep heavy growling noises – some electrical fault.
For permissible limits of noise levels for rotating electrical machines IS: 12065:1987
is being reffered.

7. What are the reasons for harmonics in the motor?

Generally even harmonics are not expected to be present in three phase motors.
Triple-n harmonics like 3rd, 9th, 15th etc. are also not expected. The dominant odd
harmonics expected are 5th, 7th, 11th and 13th etc.
Presence of strong 2nd harmonics indicates unbalance voltage, unbalance winding
impedance, rotor defects, magnetic imbalance, faulty rotor skewing etc.
Very strong 3rd harmonics indicates magnetic saturation, ground leak currents,
overloads etc. Overloading causes overheating, resulting in non-linear magnetization

which gives high 3rd harmonic winding faults, short circuits. Hot spots in rotor or
stator also may indicate higher harmonics.

8. What are the possible reasons for not coming of rated speed during start?bable
causes

a. Starting load is too high.
b. Broken rotor bars (look for cracks near rings).
c. Open primary circuit.
d. Voltage is too low.

9. What are the possible reasons for motor to take long acceleration time?

Following may the possible reasons for motor to take long acceleration time.
a. Excess loading.
b. May be rewound motor with poor quality of winding conductor having high
resistance.
c. Defective squirrel cage rotor.
d. Applied voltage is too low.

10. What are the points contributes in insulation resistance of the motor?

If the measured insulation resistance of the motor is less than 1 MΩ / kV with a
minimum of 1MΩ, when the machine is cold it is to be dried out before full voltage
is applied to the terminals of the motors and the drying out is to be continued as long
as the insulation resistance rises or until a sufficiently high value that is not less than
1 MΩ / kV with minimum of I MΩ at 75°C is reached.
While proceeding for point as above said, following factors are to be kept in mind
which affect the insulation resistance measurement. They are,
a. Surface condition.
b. Moisture.
c. Temperature.
d. Magnitude of test voltage.
e. Duration of application of test voltage.
f. Residual charge in the winding.
g. Ageing of the insulation.
h. Mechanical stresses.

11. What are the minimum recommended PI values for AC and DC rotating machines?

Following minimum recommended PI values criteria is to be followed.
a. 1.5 for class – A insulation.
b. 2.0 for class – B insulation.
c. 2.5 for class – F insulation.

12. What is the minimum recommended absorption coefficient value for AC and DC
rotating machines?

Absorption coefficient = IR value for 60 seconds = 1.3 (minimum recommended value)
IR value for 15 seconds
Tips:
a. IR value decreases some what with an increase I applied voltage. However for
machines in good condition substantially the same IR is obtained for any test
voltage up to the peak value of the rated operating voltage.
b. If the IR value decreases significantly with an increase in applied voltage it is an
indication of imperfections or fractures of the insulation aggravated by the
presence of dirt or moisture or may be due to the effects of dirt or moisture alone,
or may result from numerous other phenomena not necessarily associated with
any defect or weakness.
c. IR value for good dry winding continue to increase for hours with constant test
voltage continuously applied, however a fairly steady value is usually reached in
10 to 15 minutes. If the winding is wet or dry or dirty the steady value is usually
reached in 1 or 2 minutes after the test voltage is applied.
d. The recommended minimum IR value for AC and DC machines is determined by
the following empirical relationship.
IR = kV + 1
Where IR = recommended minimum IR in mega ohms at 40°C of the entire
machine winding and kV = rated machine voltage in kilo volts.
Temperature correction is to be applied, if winding is not at a temperature of
40°C.
e. IR of the one phase of three phases winding with other two phases earthed, is
approximately twice that of the entire winding. Therefore when the three phases
are tested separately, the observed insulation resistance of each phase should be
divided by two to obtain a value which after correction for temperature, may be
compared with the recommended minimum value of IR.

13. What is use of Tan – Delta test? And what are the recommended values?

The very purpose of this test is to detect moisture content, voids, cracks and
deterioration in the insulation and same is to be conducted on HT motors.
Based on the guidelines given in the article ‘Diagmistic testing on the winding
insulation’ by J. S. Simon (IEE vol. 127 may 1980) the contamination level of motor
winding is to be assessed from the given Tan – Delta value.
Starting Tan – Delta values Degree of contamination
0 – 4%. Low void content.
4 – 6%. Clean.
6 – 10%. Some dirt.
10 – 14%. Dirt and moisture.
14 – 16%. Gross contamination.
Question and answers Electrical Maintenance Unit
- 193 -
16 – 20%. Heavy deposit of oil dirt.
Above 20%. Severe oil and carbon contamination.

14. What are important guidelines for conducting HV test?

Based on the recommendations given in IS: 4029:1977 decided DC test voltage
= (2E+1kV) 1.6 * M
Where E = rated voltage.
2.6 = AC to DC conversion factor.
M = derator factor which is a function to be decided on the basis of the age and
condition of equipment.
The DC voltage applied in steps and the leakage current recorded at each step. A plot
leakage current Vs test voltage is to be plotted as the test progress.
Some recommendations of IS : 4029 : 1977.
a. The HV test made on the windings on acceptance shall as far as possible not be
repeated. If however a second test to be made at 80% of the voltage given by the
empirical formula given above.
b. Test voltage for completely rewound motor = full test voltage for new motor.
c. Partially rewound or overhauled motor = 75% * full test voltage for a new motor.
d. Before the test for the old parts of the winding shall be carefully cleaned and
dried.
e. Before attempting of HV DC test a minimum PI value of motor should be
obtained.

15. What are the uses of high voltage surge test?

This test gives distinct wave forms giving indications of various defects such as,
a. Turn to turn short in same phases.
b. Coil to coil short in same phases.
c. Partial phase to phase short.
d. Complete phase to phase short.
e. Improper coil connections.
f. Reverse coil connections.
g. Open coil connections.
h. Short to ground partial.
i. Short to ground complete.

16. What is the thumb rule for motor current?

Thumb rule for NO LOAD current of motors.
Type of enclosure No. of poles % No Load current of rated current
TEFC 2 15 – 20
TEFC 4 30 – 35
SPDP 2 25 – 30
SPDP 4 35 – 40
SPDP 6 to 8 50 – 55
SPDP 10 80
Note: TEFC (Totally enclosed fan cooled) motors are low inductive having low
active material in comparison to SPDP(Screen protected drip proof) motors.
Thumb rule for calculating positive sequence and negative sequence current in
motors.
a. Positive sequence current: Average of all three phases currents.
b. Negative sequence current: Maximum deviation of any of the phase currents from
the average.

17. How you evaluate the insulation condition based on PI value?

Evaluation of insulation condition based on PI value
PI value Insulation condition Recommendation
1.0 – 1.5 Bad Drying is mandatory
1.5 – 2.0 Doubtful Drying is recommended
2.0 – 3.0 Adequate No drying is needed
3.0 – 4.0 Good No drying is needed
> 4.0 Excellent No drying is needed

18. What are the conditions monitoring for the motor bearings?

Bearing oil analysis is a useful tool in determining bearing performance and possible
deterioration. Periodic checks for oil colour, viscosity and acidity can aid in
preventing or anticipating bearing failure.
Oil analysis tests
Symptoms Possible cause Test Cost
Viscosity Water or high Water content Low
temperature ASTM 445 viscosity Low
ASTM 974 neutralization number Low
ASTM 664 neutralization number Moderate
Viscosity change
colour change
Oxidation
ASTM 2296 neutralization number Moderate
Spectroscopy Low
Particle count Moderate
Direct reading ferrography Moderate
Particles Bearing
deterioration or
foreign matter
Analytical ferrography High

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