40 ,63, 80 or 100A RCD

[davetheglitz]

Why do RCD's come in so many different current ratings? I would have thought that it would be easier and cheaper for manufacturers to do a standard 100A RCD for all applications given the small amount of extra copper for the thicker coil windings and contacts and the smaller tooling costs - or have I missed something?

 

[green-hornet]

Its mainly for discrimination and is mostly used on industrial boards or larger commercial premises.

 

[batty]

In the old days there was quite a bit of difference in price between the lower amp ones and high amps ones but nowadays because they are so cheaply made as you say they may as well make just one size and quite often lower amp ones can be dearer than high amp ones.

 

[sparkytim]

As the rating just relates to current carrying capacity and NOT any over current fault protection (other then RCBOs) then discrimination is not involved is it?

?:|

 

[green-hornet]

As the rating just relates to current carrying capacity and NOT any over current fault protection (other then RCBOs) then discrimination is not involved is it? ?:|

Yep, just read the post properly, my mistake was to think of them in mA and not current, thanks for pointing that out (to me anyway)

 

[sellers]

Its mainly for discrimination and is mostly used on industrial boards or larger commercial premises.

Isn't the rating just for the max load of the switch not fuse rating, therefore discrimination doesn't apply to this?

Aren't you thinking of an RCCB? Please correct me if I'm wrong with all your years of wisdon GH ?:|

 

[green-hornet]

Yes as I have realised my mistake in post 5 ,pointed out to me by post 4

I did think (wrongly) that the question was about mA and not current, but its sorted now.

 

[sellers]

I started doubting myself then. Thanks GH

 

[sparkytim]

Thats ok but just to add to it, I undestood the only way to provide discrimination between RCDs is to provide time delay on up stream if more then 1 on circuit.

 

[green-hornet]

Yes and I have tried, it never quite works out the way you want when large induced faults are sent down the line, 9 times out of ten nearly all RCD's trip at once.

 

[sellers]

In what applications would you need a few RCD's on an installation?

They're only really useful on final circuits with RCBO's. I remember working on a museum in wigan with very old crabtree boards with a RCCB on the incommer. Managed to trip it - twice!

 

[sparkytim]

Application could be where Distribution circuit requires RCD protection for some reason with RCDs on final ccts downstream.

i.e. Supply to shed on a TOTAL TT system with 30 m A RCD in shed.

 

[spinlondon]

RCD discrmination is most often used in TT systems, where the distribution circuit is on a 100mA time delayed RCD, and circuits from the CU have 30mA RCD protection.