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Primary protection of transformers. For the primary I always simply said: The control fuse/circuit breaker with a current rating of minimum transformer rating = 1.3xImax that’s it. The thinking behind this is: We normally install overload protection on the secondary and only short circuit protection on the primary. The factor of 1.3x accounts for the high inrush currents associated when powering up. Until I started digging a little bit and it got all complicated. Different takes: Option 1: I put this first – because then you don’t have to read the rest. RTFM – If you don’t know the acronym it’s ok. Look in the manufacturer’s manual or on the transformer. Good manufacturers will recommend the correct size and type of fuse to protect their transformer. This also ensures obligations for any guarantees/warranties from the manufacturer. So before you walk out that electrical shop…put the fuse+holder /circuit breaker on the same invoice. Option 2: Purchase a special fuse for transformer protection. This is most important if some compliance to a project specification is to be achieved. These Tr class fuses are selected as per the transformer rating because its design accounts specifically for transformer currents. But I wish you luck to find this at the normal electrical outlet. Option 3: The ultimate. Get the transformer starting surge curve from the manufacturer and match it to the smallest slow blow fuse curve (This is if you are like me – don’t mind reading graphs). The transformer curve should not intersect with the fuse tripping curve. But let’s shortcut here and see what interesting manufacturers say:
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