HT AND LT BREAKER TESTING
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
- 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 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
- Display
- 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 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.