Lightning Arrester Selection

I would refer to the power utility for the location this transformer will be installed for advice. All of your concerns are related to your local weather and local power grid configurations.

Check with the supply utility as in Aus the transformers come connected with primary and secondary surge diverters/lightning arresters. If you wish to protect your incoming supply then they should be able to advise you on the KA rating and class of arrester you can use on your incoming supply. There are also three phase units made by a number of manufacturers to help suppress the incoming spike and I have installed a number of these units in the past on buildings to protect the computer systems and office equipment.

The main thing to find out is if the line you are fed from can be considered effectively grounded. We normally assume ungrounded for lines like that, so your line protector steady stare voltage is equivalent to line potential, 11kV to ground for surge arrestor steady state conduction withstand voltage without damage. This varies with the surge protector design, and how important this transformer is to your operation. Experience and budget will dictate the amount of energy can be transferred when the voltage exceeds the conduction threshold. More capacity, lower likelihood of transformer damage.

You can get some estimate from the utility of how long a low current ground fault could persist on their line, overhead lines can flash and be hit multiple times to ground, but you typically can’t count on covering all possible situations.

A <...75kVA, 3 phase 50Hz, 11kV/415V pole mounted transformer...> is an unlikely object to <...have...> in the private domain, as few in that domain will have the necessary skills to deal with the <...11kV...>, making this a very strange and alarming post. Merely asking the question in an open forum convinces readers that the necessary skills and training are not to hand.

In all cases of <...Should...check...>, open a dialogue with the <...electric supply company...>, and have its trained staff handle all of it, for safety's sake, as getting it wrong involves half a day out with the local Undertaker and the remainder a lifetime of sorrow for loved ones. A suitable telephone number will be found on the latest bill for payment from that organisation.

If the above is to be ignored, then please ensure that all other CR4 readers are some distance away during the work activity.