The basics of outdoor transformer lightning arrestors

[bdn2004]

We're installing a new oil filled transformer and connecting it with overhead open conductors, it is coming with lightning arrestors.

I understand they only conduct when there is a high voltage surge in the line and then the voltage is diverted to ground. Say this happens, doesn't this cause a phase-to-ground fault, and with our relays it trips the line?

And if it takes a hit, how do we know? And can the arrestors be re-used? How do we know they weren't destroyed?

Also the transformer mfg is recommending that each lightning arrestor be connected individually to one ground rod with an insulated wire. I notice on all of our otherinstallations that the arrestors are conncected with looks like a #2AWG bare wire. Any reason for this?

 

[magoo2]

A surge arrester is like a voltage sensitive switch. When the voltage reaches critical point, it conducts to ground. After doing that, it returns to acting more like an insulator.

If an arrester fails due to energy above its capability, it will usually blow an isolator, which disconnect the ground lead from the arrester.

There are some cases in which the isolator doesn't blow, so you don't know that you may have a bad arrester until the next lightning hit takes out the transformer.

Modern day MOV arrester have very high reliability. As a result, you shouldn't have many instances where the arrester fails and doesn't blow the isolator.

We purchased an arrester tester from a transformer manufacturer about 20 years ago. This applies the MCOV rating to the arrester and records the current drawn by the arrester. This is not at all portable, so we use this when transformers come back to our shop. There might be other arrester testers on the market today so you might want to check around.

 

[odlanor]

magoo2,

...it conducts to ground. After doing that, it returns to acting more like an insulator.

I guess your definition is not good. If were like insulator it will be result in a short circuit to ground.

A surge arrester is like a voltage sensitive switch with variable resistance. When the voltage reaches critical point,resistance to ground is low and it conducts to ground. When the voltage returns to normal resistance to ground is high and it cut current path.

 

[magoo2]

odlanor - I think you're hung up with semantics. I used the analog of a switch because it is easy to comprehend.

Under normal operation, an arrester behaves like an insulator (open switch) in that it has a high impedance to ground.

bdn2004 - As far as the ground lead, we use a #4 bare copper wire. The wire doesn't have to be insulated and insulating it provides little value.

Also, if we have a three phase overhead installation, we connect the ground leads together and run one ground wire down the pole to the ground rod.