I work on equipment that runs off 3 phase 208V but it uses uses a transformer to drop it down to 120V for most of the controls. On this equipment I noticed that there are two fuses on the lines exclusively feeding the 208V side of the transformer and a fuse directly off of the hot side on the 120V side of the transformer.
Isn’t the fuse on the 120V side of the transformer redundant? From my understanding, if there is a current spike on the 120V side of the transformer then that will cause a current spike on the 208V side of the transformer and immediately blow those fuses anyways. Is this just a certification thing where that redundancy is required? I’m in the US but this equipment does also get shipped to various overseas locations. Also, while it isn’t standard, this equipment is capable of passing a TUV inspection if a customer requests it so I’m not sure if the potentially redundant fuse is just a TUV requirement.
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My Electromagnetics is a bit rusty, apply a healthy measure of skepticism:
I’d say the two fuses are there to protect the transformer itself.
In the example you use, the upstream of the transformer would be protected by the 280V fuses but the transformer itself will have to suffer the overcurrent, and could be damaged by it. If the transformer is dipped in cooling oil, the overcurrent could even lead to an explosion.
As for the TUV inspection, the two fuses would be desirable for any TUV cert, and they would be required if there is any chance of explosion.
Definitely no chance of an explosion here. I’m not sure what the transformer is rated at off the top of my head but it is fused at 3A on the low voltage side and 2A on the high voltage side and I know it is rated for well over that. It’s a pretty beefy (5-10lb air cooled) transformer for the all of 0.5-1A it actually typically supplies on this machine.
I am actually leaning towards TUV being the explanation though because your comment just reminded me that we actually got dinged by them before on a different piece of equipment because we didn’t have fuses on the DC side of a DC power supply despite the AC side having a fairly restrictive fuse directly upstream of that power supply which should have prevented any catostrophic failure on its own. I’m willing to bet the rules are the same for transformers.