16, 32 and 63 as a rating

[Cookie]

Why are 16, 32 and 63 amps used as an MCB rating instead of 15, 30 and 60 amps? Best theory that I can come up with was when the UK switched to 230 volts this was to make up for the drop in voltage but have noting to back that up.

Here are the ratings I'm familiar with:

https://imgur.com/a/trtK7pI

View: https://Upload the image directly to the thread.com/a/trtK7pI


The nominal voltages which go along with them:

https://imgur.com/NFO74Tp

View: https://Upload the image directly to the thread.com/NFO74Tp

 

[Cookie]

The way it was explained to me is that the series of values aids in coordination and discrimination between breakers.

If you take the series
16, 20, 25, 32, 40, 50, 63, 80, 100
then from each point, if you go up TWO steps, then that value is ~1.6x where you started, and your downstream breaker should now reliably trip before the upstream.

IF you're in the thermal region, right?

 

[pc1966]

Yes, the thermal curves provide selectivity, but once you hit the magnetic trip there is very little.

But for fuses it is generally OK for BS88 style at a 1:1.6 sort of ratio, but to be sure you would normally go for a 1:2 ratio. For example, if you have a 80A DNO fuse then a 40A fuse-switch to feed something (out building DB, whatever) then wherever happens on the end of that won't take out your main supply.

Getting selectivity MCB to fuse is hard as well, but often if you have a 1:2 ratio and B-curve you get a usable level, e.g. 20A B-curve & 40A fuse is going to be to PFC of around 900A which, depending on cable length, might never be reached so the fuse lives past a MCB trip on a hard fault.

 

[Cookie]

Yes, the thermal curves provide selectivity, but once you hit the magnetic trip there is very little.

But for fuses it is generally OK for BS88 style at a 1:1.6 sort of ratio, but to be sure you would normally go for a 1:2 ratio. For example, if you have a 80A DNO fuse then a 40A fuse-switch to feed something (out building DB, whatever) then wherever happens on the end of that won't take out your main supply.

Getting selectivity MCB to fuse is hard as well, but often if you have a 1:2 ratio and B-curve you get a usable level, e.g. 20A B-curve & 40A fuse is going to be to PFC of around 900A which, depending on cable length, might never be reached so the fuse lives past a MCB trip on a hard fault.

Good to know.

RK-1 and RK-5 fuses will achieve 100% selective coordination up to 200,000 amps provided a 2:1 ratio.