It's not uncommon to find the arrestors in the transformer zone. Given that the arrestors are almost always mounted on the transformer and that any transformer diff zone that runs breaker to breaker will include the arrestors it has to be quite common. Transformer differentials are never quite as fast as bus diffs and are less likely to false trip for an arrestor than would a bus diff. Better to have them in the transformer zone than the bus zone.

In such cases its safe to just set percentage restrained differential and other similar parameters as though the arrestors are not present?

Reason I ask because I see many cases where the transformer's own CTs are used for differential (basing my values off that) while the breaker to breaker zones are protected separately. And yes- that would be a belief. You won't believe how many older stations simply have a disconnect from the bus going right to the transformer putting the LAs in the buss zone.

Mbrooke (Electrical),

We have used the breaker to breaker differential protection for the transformers and the lightning arrestors (LA) in the differential zone. As such steady state of an arrestor is in terms of milli amps. Thus the relay is not sensitive to this leakage current.

When the discharge, it is active for few microseconds only. That means we are talking about the voltage/current burst for the period of microseconds which when converted to frequency it will be in MHz. These CTs are not sensitive to pick up such bursts. Hence such practice is quite safe.

Krisy's, thank you, this is what I wanted to hear! I've seen a bunch of installs where the differential is applied only within the transformer tank- I wondered if that was just to pin point a fault on the outside or if the LAs were driving it which made me question the un-delayed settings used when breaker to breaker differential was involved. Guess not then.