Switchgear Questions & Answers

Switchgear

1. Mention the commissioning tests on breaker and bus bars.

Breaker

a. Milli volt drops test between the interrupting contacts and between the isolator
contacts.
b. Closing and opening timing of the breaker for 5 times.
c. Checking whether the breaker trips or closes when the logics are fulfilled.
Bus bars

a. Milli volts drop test for the contact resistance value.
b. Tightness of the joints.
c. IR values between phase to phase and phase to ground.

2. Explain clearly the three positions in 415 V breaker.

a. Service position: Power connections and control connections are available to the
breaker.
b. Test position: Power connections are cut off but control connections are available
to the breaker and it can be tested.
c. Disconnect position or rack out position: This is for maintenance of the breaker
and in this positions both the control and power connections are not available.

3. What do mean by trip free system in breaker?

In trip free the breaker is free to trip at any time. If both close and trip signal is
present at same time (instant) the breaker will attempt to close and positively trip.
When the breaker trips it will not close again even if closing signal exists because of
anti pumping feature.

4. What is the purpose of spring charging in 415 V breaker?

If the breaker is to be closed and tripped manually the closing time and tripping time
would vary from person to person. Also it would not be very fast. So spring charging
is provided. It gives uniform timings irrespective of the operator and its action is fast
and closing and tripping time is very less.

5. What are the built in protections provided in 415 V breaker?

a. DINF (making current magnetic release)
This consists of a laminated magnetic circuit. This is placed under the lower
current terminal pole. This is provided for all the three poles. It has a core that
rotates in the air gap. It is held by spring. During protection the magnetic forces
developed overcome the spring tension and the core is attracted. The mechanical
force developed is used to trip the breaker. This protection acts during the closing
of breaker if any fault exists. The current is set to 5 times the rated current.
b. DIRS (short time magnetic release)
The construction is same as DINF, but it has a mechanical timer, which can be set
accordingly. This protection acts when any fault comes during breaker in service.
The current rating is set to 3 to 8 times the rated current.
c. DIT – S (thermal over load protection)
This consists of a three bimetallic strip, which gets heated up when over loaded
and trips the breaker by a lever. It is placed in front of the breaker. Setting range
is 60% to 100%.

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