Testing Of VCB

1.CRM TEST

Measurement of resistance of main circuit or contact resistance test (CRM Test)

Introduction

Introduction to contact resistance test Contact resistance test is performed for Circuit breakers which are very important entity of Electrical power system. It plays a key role in protection of electrical equipment. It conducts current during normal condition. However in faulty condition, it interrupts the power/current by opening its contacts. When the fault is removed, continuity of electrical system is restored by making contact of circuit breaker. For this to happen reliably and smoothly, the contacts of circuit breaker should be in good condition. There should make and break properly. There should be no carbon or any other unwanted material on its surface. The contacts should be properly lubricated. With the passage of time, contacts of circuit breaker worn out. Since the contacts of circuit breaker are not visible therefore contact resistance testing is necessary to check condition of contacts of circuit breaker. It is also necessary to select correct circuit breaker so that the contacts lasts longer. Proper selection is explained in article, how to select circuit breaker .

Damage to contacts of circuit breaker

Due to the making and breaking of contacts at high speed and pressure, the contacts can be damaged.
Due operation of breaker, sparking occurs which can depreciate the contacts.
Sparking results in carbon formation which accumulates on the surface of `contact and can increase in its resistance.

Apparatus for contact resistance testing

To perform test according to standard contact resistance test procedure, micro ohmmeter should be used. It is shown in the figure
Micro ohmmeter must have the following provisions
Should supply as high current as 200A.
Should have the provision of varying current
Current meter.
Display resistance.
There are two types of leads or connection wires in resistance test set. One set of leads is called current leads and other is called sensing leads or voltage leads. The purpose of current leads is to provide high current while that of sensing leads is to provide voltage.

Why You Need Contact Resistance Test?
The contacts in the circuit breaker needs to checked periodically to ensure that the breaker is healthy and functional. Poorly maintained or damaged contacts can cause arcing, loosing phase, and even fire.
This test is especially important for contacts that carry large amounts of current (e.g. switchgear bus bars) because higher contact resistance can lead to lower current carrying capacity and higher losses. Ductor testing is usually performed using a micro/milli-ohmmeter or low ohmmeter.
Measurement of the contact resistance helps in identification of fretting corrosion of contacts, and allows contact corrosion to be diagnosed and prevented. Increase in contact resistance can cause a high-voltage drop in the system, and that needs to be controlled.

2. Timing test Introduction to breaker timing test

Circuit breakers are one of the important and critical part of electric power system. Circuit breakers are the backbone of electrical protection system. Because of their key importance, they must be checked periodically. Circuit breaker timing test is among different maintenance tests which should be performed regularly. It measures the mechanical operating time of the circuit breaker. This test is always important because delay in operation of circuit breaker can have disastrous effects on the equipment when fault occurs.

Operation time: Operation time of the circuit breaker is the closing and opening times of contacts. Definition of these operation times is presented by IEC 56.3.105 as follows. breaker
Closing time: The time interval between energizing the coils of the breaker (when CB contacts are in open position) and the instant when the contacts touch each other.
Opening time: It is opposite to that of closing time. It is the time interval between energizing the trip coil (CB contacts are in open position) and the instant when contacts separate from each other.

Importance of timing test of circuit breaker

circuit has a key role to play in the protection system. The correct operation time of circuit breaker is important due to the following reasons;
Minimum damage during fault. When fault occurs, relay senses the abnormal condition and gives a trip command to the circuit breaker. The circuit breaker after receiving trip signal, energizes the trip coil and open its contacts thus detaches the faulty part of system from rest of system. The circuit breaker takes some time to do the process. The equipment’s installed in the power system are designed such that they withstand high current (other abnormal condition) for very limited time. If the circuit breaker failed to operate in that limited time, the equipment may be damaged or destroyed. Therefore it is very important that circuit breaker must operate in specific minimum time (operation time) in order to minimize damage. This feature makes it very important to check the operation time of circuit breaker periodically.
Relay coordination. Circuit breakers are installed at different positions to provide protection to power system. It is recommended that only faulty part of power system should be detached while the other should remain intact. It is only possible that breaker near to the fault should be tripped while breaker away from load and near to source should remain intact. This is called relay coordination. The timing of the relay are set in such a way that breaker near the fault should trip first. However if the breaker fails to operate in its specific time, the breaker away from fault (near to source) will trip hence healthy part of system will also be detached. So correct operation time is necessary for the relay coordination.