Consider a 230V transformer with whatever voltage on secondary. The core is connected to the earth wire as per regulations.If primary insulation fails and touches the core, fault current starts to flow and trips the breaker. So far so good.But what if secondary fails, but not primary, so the earth wire suddenly becomes an open circuit under voltage? Won't every piece of earthed equipment in your home become live? I mean, with 12V on secondary it's not that big a deal and nothing would happen until you provided a return path to the secondary anyway, but still, how do you protect yourself from that kind of situation?
>>1510014i'm not shure if i understood your situation correctly but you can "earth" one leg of the secundary. for example when i build amplifiers the tranny core goes to ground AND also the secundary winding is grounded (i normally use a "center tap"on my tranny)
>>1510014>so the earth wire suddenly becomes an open circuit under voltageDisregarding this part, I think you meant shortedThe secondary is floating if the secondary circuit is ungrounded. Ungrounded, there is no meaningful way to measure the voltage referenced to ground. It would have a static voltage like an ungrounded piece of metal suspended in the air. If you had a 12Vac secondary, whichever lead you touch with your finger would becone a ground reference and the other lead would become 12Vac when referenced to ground.That said. a secondary short to a grounded core would act as a grounded center tap, giving the secondary leads meaningful measureable voltage when referenced to ground.If the secondary circuit is grounded, then it could lead to excessive current in the secondary coil which would cause excessive current in the primary
>>1510014If it fails open, you will have no current in your secondary. Without a current path in the windings, the changing emf around the core will not be able to produce much of a voltage, and the small floating voltage you do get will not be able to deliver much current so this kind of fault isn’t very dangerous even for higher kVA transformers. If it shorts to ground inside the winding, your primary protection “should” trip. Sometimes they don’t and you let the magic smoke out.