Application Problems and Solutions for MCB and MCCB in Electrical Circuits

Kathmandu : Under the weekly educational column "Energy Knowledge" being published every Friday by Urja Khabar, this week we have brought about problems and their solutions for MCB - MCCB, which is the commonly used gadgets in electric meter box. Multiple choices questions on different topics have also been included. The last time we had talked on transformer failure- its causes, remedies and preventive measures.

Common Problems and Remedies

1. The MCB at meter box or elsewhere trips immediately after being on. Why?

Answer:

(a) The wiring may be short circuited.
(b) In case the MCB trips after some time, it implies that the capacity of MCB is less. Its replacement with higher capacity MCB may resolve the issue, but ensure the wiring is adequate for the higher capacity.
(c) Long and thin wires (e.g., red/black) used in barns or distant locations may have high resistance, causing a delayed trip after short-circuiting.

2. Why does a tester glow when the heater coil is touched after switching it off?
Answer :

(a) The heater switch is connected to the neutral instead of the phase wire.
(b) If the tester glows dim, the neutral earthing of the transformer may be poor or deteriorated over time.
(c) Excessive flow of current in the neutral of the transformer will also cause the tester to glow dimly.

3. Why does the MCCB of a transformer trip frequently?
Answer:

(a) Due to overload. This can be checked by using a clamp meter.
(b) Even if the overall load is balanced, an imbalance in phases may cause tripping. Measure the current on each phase using a clamp meter to identify the problem.
(c) This may also be due to a short circuit in the LT line. Check the wiring for damage.
(d) The rubbing of sagging LT lines against each other due to wind or birds can be another cause of MCCB tripping.
(e) Overheating due to sparking at loose contacts in MCCB cables can also cause it to trip even under low-load conditions.

Multiple Choice Questions

(1) What is the function of transformer oil?

(a) Dielectric and cooling
(b) Cooling and lubrication
(c) Lubrication and dielectric
(d) Insulation, lubrication, and cooling

Answer: (a) Insulation and cooling

(2) What is not to be missed during transformer inspection?

(a) Dropout fuse
(b) Oil level
(c) Silica gel
(d) Cleanliness

Answer: (b) Oil level

(3) What does an ideal transformer not change?

(a) Power
(b) Voltage
(c) Current
(d) None of the above

Answer: (a) Power

(4) On the principle of which a transformer works?

(a) Mutual induction
(b) Electromagnetism
(c) Dynamically induced EMF
(d) Energy transformation

Answer: (a) Mutual induction

(5) Which of the following transformer types the secondary circuit should not be kept open while the primary circuit is on?

(a) Auto transformer
(b) Potential transformer
(c) Current transformer
(d) Power transformer

Answer: (b) Potential transformer

(6) What kind of fault does a differential relay in a transformer detect?

(a) Internal faults
(b) External faults
(c) Overcurrent
(d) Overvoltage

Answer: (a) Internal faults

(7) In a delta/star distribution transformer, under load conditions

(a) Primary current is higher than secondary current.
(b) Primary and secondary currents are the same.
(c) Primary current is less than secondary current.
(d) None of the above.

Answer: (c) Primary current is lower than secondary current.