Circuit breakers - are you using the right micro-ohmmeter?

Damon Mount - UK power manager

One of the most common uses for micro-ohm meters is to check high and medium voltage circuit breaker contact resistance. Circuit breakers come under a great deal of strain as they are required to break a high voltage circuit, often under load. The arc that is generated from the switching action can damage the face of the contacts, and this could result in a catastrophic failure of the breaker or other substation equipment downstream.

Circuit breakers are a critical piece of substation infrastructure. They are used to switch parts of the network in and out for isolation purposes such as for planned maintenance, or to remove the power in the event of a fault as part of the protection system.

If they fail to operate there can be serious consequences for the rest of the network

Arcing at the contacts creates carbonised layers which over time will build up and the live contact area will reduce or become pitted, leading to an increase in resistance and heating. This situation reduces the efficiency of the circuit breaker and can lead to failure in an active transmission or distribution system which may result in the loss of a substation.

As the contact resistances should be very low, probably 10s of micro ohms, a very accurate micro-ohmmeter with a high resolution needs to be used to obtain an accurate reading of the resistance between the contacts when the breaker is closed.

The carbonised layers may only add a small amount of resistance but the consequences of this can make a huge difference in the performance of the breaker. Excessive heat can actually weld the contacts together and this may result in the circuit breaker not operating at all when called upon to do so!

10A or 100A?

Let’s face it, it’s easier to take a battery powered micro-ohmmeter to site to do the tests. No messing around with power supplies and long leads or dragging a heavy generator to an inaccessible substation. So for years, many Engineers have been using battery powered 10A micro-ohmmeters such as the DLRO10 to do this type of testing as it’s convenient and does the job. But does it really?

High voltage circuit breakers operate at a higher current than they are tested at in this case. They might not react in the same way as when they see current nearer the operating level. The carbonised layers may resist an injection of 10A but pass a higher test current, leading to differences in the readings. This can lead to the test failing the circuit breaker unnecessarily. The stability and reliability of the readings increase with the higher current as the very small voltages being measured are more reliably captured.

This issue is recognised and both IEC and ANSI standards recommend higher test currents to test circuit breaker contact resistance; 100A as a minimum for ANSIC37.09 and for IEC62271-100, any test current value between 50A and the rated current of the circuit breaker.

Inconveniently, most high current micro-ohmmeters are 100A, 200A or even 600A mains powered units, which need the associated power supplies or generators to power the instrument to produce the test current required. It is partly this time consuming set up that has limited the adoption of high current circuit breaker testing.

Well, now there are two solutions to this problem.

You want small? Megger has a hand held battery powered 200A micro-ohmmeter specifically designed for circuit breaker applications that uses a supercapacitor to inject a high current for a short duration to give a highly accurate reading in a class leading package.

You want flexibility? The new Megger battery powered DLRO100 is designed for multi-use applications including circuit breaker contact resistance and provides up to 100A test current with the capability of supplying constant current for tests that require a longer duration.

So there is now no practical excuse not to use high current micro-ohm meters designed specifically for testing circuit breakers. These new units provide circuit breaker testing to the recommended international standards, while having the convenience and flexibility of go anywhere battery power.

To download your FREE copy of the Megger guide to low resistance testing click here