We do not supply these. This page is here because the question comes up and the answer is hard to find written plainly. What we do supply is three phase and single phase pad mounted distribution transformers from 25 to 2500 kVA.
What it does
A potential transformer, also called a voltage transformer, steps a system voltage down to a standard instrument level, almost always 120 volts at nominal. A 12470 volt system with a 120 volt secondary uses a ratio near 104 to 1. The meter or relay is then working with a safe, standardised voltage regardless of the system it is watching.
Why it is not a distribution transformer
Capacity. A PT is built to supply a few tens of volt amperes, enough for an instrument, and nothing more. Connecting any real load to it will burn it out. A distribution transformer of the same primary voltage is a thousand times larger in capacity and built for an entirely different duty.
Fusing
PTs are fused on the primary with small current limiting fuses, and frequently on the secondary as well. The primary fuse is there to clear the PT itself if it fails, without taking down the system it is connected to. Since a PT failure on a medium voltage bus is otherwise a fault on that bus, the fuse is not optional.
Common questions
What is a potential transformer
An instrument transformer that steps system voltage down to a standard level, almost always 120 volts, so that meters and protective relays can measure a medium or high voltage circuit safely.
What is the difference between a potential transformer and a current transformer
A potential transformer scales voltage, a current transformer scales current. Both are instruments rather than power supplies, and both exist so equipment can measure a circuit it could not safely connect to directly.
Can you power a load from a potential transformer
No. A PT is rated for a few tens of volt amperes, enough for an instrument burden. Connecting a real load will destroy it and may cause a fault on the system it is fed from.
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