S
sparkmanuk
I postedthis question about two weeks ago,
If you have a busbar chamber supplied by a large cable fused at say 100A, and then smaller cables taken from the chamber that go to isolators that are rated at 30A. Technicaly the smaller cables are underated but, cannot pull more than their design current due to the isolators they are connected to, is this allowed by the regs, I know this is done all the time but is it BS7671.
The concensus of posts was it is allowed!, I would like to add further debate to this, by saying that if this is allowed then you could have say a 4mm swa on a 63A breaker out of a consumer unit as long as it is terminated into a fused isolator that is the proper rating for the cable, if i am correct Part p has stopped spuring off the domestic socket ring to a double socket in 2.5, since 26A could theoretically be pulled, I know part p dosent cover all industrial.
I am wondering if their is not a written rule or interperitation of another rule that lets a cable have a higher protection rating than the cable can handle, or a certain percentage higher fuse rating for a cable as long as it is terminated to a fuse isolator of proper fuse rating.
Because as an example if you have a busbar chamber protected by a 200A fuse with say a 2.5mm swa going to a 20A isolator and a 4mm swa to a 30A isolator and a 10mm to a 60A isolator, if one of the cables were to be squashed against the wall by say a fork lift or object.
Then the smaller calbles are more likey to blow on impact, so my point is the smaller the cable the greater the chance of a 200A explosion which makes me think their is either a rule of thumb or written guidence somewhere, like say steel wired armour csa to potential dead short current, and i know you could say you design the installation to the risks but the use of a factory/ installation can change.
I would welcome anyones thoughts on this.
If you have a busbar chamber supplied by a large cable fused at say 100A, and then smaller cables taken from the chamber that go to isolators that are rated at 30A. Technicaly the smaller cables are underated but, cannot pull more than their design current due to the isolators they are connected to, is this allowed by the regs, I know this is done all the time but is it BS7671.
The concensus of posts was it is allowed!, I would like to add further debate to this, by saying that if this is allowed then you could have say a 4mm swa on a 63A breaker out of a consumer unit as long as it is terminated into a fused isolator that is the proper rating for the cable, if i am correct Part p has stopped spuring off the domestic socket ring to a double socket in 2.5, since 26A could theoretically be pulled, I know part p dosent cover all industrial.
I am wondering if their is not a written rule or interperitation of another rule that lets a cable have a higher protection rating than the cable can handle, or a certain percentage higher fuse rating for a cable as long as it is terminated to a fuse isolator of proper fuse rating.
Because as an example if you have a busbar chamber protected by a 200A fuse with say a 2.5mm swa going to a 20A isolator and a 4mm swa to a 30A isolator and a 10mm to a 60A isolator, if one of the cables were to be squashed against the wall by say a fork lift or object.
Then the smaller calbles are more likey to blow on impact, so my point is the smaller the cable the greater the chance of a 200A explosion which makes me think their is either a rule of thumb or written guidence somewhere, like say steel wired armour csa to potential dead short current, and i know you could say you design the installation to the risks but the use of a factory/ installation can change.
I would welcome anyones thoughts on this.
