06-03-2016, 08:58 PM
In the old days in the UK a small appliance would likely be connected to a 5A rewireable fuse if you were lucky. Otherwise a 15A fuse.
Realisitically the plug fuse can only provide short circuit protection. In other words we hope the plug fuse blows before the cable blows. Even a thin mains flex should survive a 5A fuse blowing due to short circuit. I suspect a thin flex would "protect" a 16A MCB. It's all to do with the I^2 * t ratings of both the fuse/MCB and the cable. It may also depend on the loop impedance. A long piece of thin flex will limit the short circuit current considerably, thus reducing peak I but increasing t before the fuse blows.
Overcurrent protection is another matter. I think I more or less showed in a previous post that even a 1A fuse will provide pretty meagre protection against overcurrent. For the simple reason that even under fault conditions such as shorted turns the current is unlikely to be high enough to blow a 1A fuse quickly if at all.
Terry is entirely correct to state that the manufacturer is responsible for providing additional protection if needed. But remember we are often dealing with vintage equipment where designers didn't always get it right.
Realisitically the plug fuse can only provide short circuit protection. In other words we hope the plug fuse blows before the cable blows. Even a thin mains flex should survive a 5A fuse blowing due to short circuit. I suspect a thin flex would "protect" a 16A MCB. It's all to do with the I^2 * t ratings of both the fuse/MCB and the cable. It may also depend on the loop impedance. A long piece of thin flex will limit the short circuit current considerably, thus reducing peak I but increasing t before the fuse blows.
Overcurrent protection is another matter. I think I more or less showed in a previous post that even a 1A fuse will provide pretty meagre protection against overcurrent. For the simple reason that even under fault conditions such as shorted turns the current is unlikely to be high enough to blow a 1A fuse quickly if at all.
Terry is entirely correct to state that the manufacturer is responsible for providing additional protection if needed. But remember we are often dealing with vintage equipment where designers didn't always get it right.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
