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Posted By Topic: RCD selection a

Apr 02 2019 15:13

Now I believe this is an age old question but I just want some clarification

According to "The load current rating of an RCD shall be not less than the greater of the
(a) the maximum demand of the portion of the electrical installation being
protected by the device; or
(b) the highest current rating of any overload protective device on the
portion of the electrical installation being protected."

Now b is easy to understand, it can't be lower than any of the MCBs but a has me confused

How are we calculating this maximum demand?

I thought basically MD in final sub circuits only really worked with assessment or limitation which would basically mean in most situations and GPO controlling MCBs limitation kicked in and that the sum on the MCBs would have to be lower than the RCD for example 3 20a MCBs could not be on a 40a RCD but 2 16a and an 8a could

Now that all made sense but I had seen this point argued by the fact that it should be calculated treating it as a submain for that portion and the MD calculated as so which would make this ok but wouldn't that mean that the RCD that's only rated for 40a could actually be overloaded with more than that? Eg 3 20a MCBs could have 14a-19a running though them and none of them would trip but it would be higher than the RCD is rated for

Am I missing something? Or is this just ok because it's unlikely to happen? If anyone could shed some info on there reasoning and the way they see it that'd be much appreciated


Apr 02 2019 17:12

For your example of 3 x 20 A mcbs - or any other combination - you can arrive at the MD for the (up to) 3 subcircuits by any of the four permitted methods.
For example, you could use limitation, in which case the MD will be the sum of the 3 mcb ratings..
But you can also use app C's calculation; eg
Ct 1: 8 x 10 A GPO = 10 A
Ct 2: 10 x 10 A GPO = 10 A
Ct 3: 9 x GPO = 10 A
Total MD for part of installation to be protected = 30A.

True in rare circumstances the RCD might be loaded beyond its rating, but - just like "protecting" those 10 A sockets with a 20 A mcb - we're not required to protect every fitting against possible overload. So it's acceptable to use a 40 A or even a 32 A RCCB to protect this group of subcircuits.

What we are required to protect against overload is the conductors, so for those we can't use MD; have to use a conductor size that suits the protection, generally either the supply fuse (upstream) or the sum of 3 mcbs (downstream). So minimum CCC for the RCD feeds (A & N) is 60 A.

Apr 02 2019 17:19

Thank you once again AlecK

That basically clears it up for me then, I had seen you or someone else mention that calculation was fine but just because I'd put some extra thought to it it didn't seem right that you could potentially overload the RCD got me thinking but I guess because in this situation it's a minimal chance it's an acceptable risk

Apr 02 2019 17:28

By way of an example of the (a) provision.

Soon after the AS/NZS 3000:2007 edition was released in Australia, many thought they could save a couple of dollars by using TWO 25 A RCCBs in a domestic installation to supply the multiple MCBs and final sub circuits required to be protected by RCDs.

The MD of the entire installation was worked out using Appendix C table C1 and the MD was assessed at not exceeding 50 amps, so TWO 25 A RCCB should have been OK assuming the load was half the MD max on each RCCB.

What happen in actual installations was that one or both RCCBs where being burnt out from over current so the MD division of half the MD on each RCCB was NOT occuring in practice.

The RCCD manufacturers refused to replace them under warranty as they claimed there was no overcurrent protection being provided for each RCCD, so the electrical contractor ended up bearing the cost of the replacement RCD due to poor design.

I note in NZ that some manufacturers do NOT market 25 A RCCDs based on these AU events.

The best solution is to use an RCBO on each final subcircuit that requires RCD protection; an RCBO is self protecting for over current; OR
Use 40 or 63 amp RCCBs so that one RCCD could cover the MD max on its own if required, hence they is why the RCCD needs to be rated for the max MD that could flow under actual working conditions.

Apr 02 2019 18:12

Interesting, thank you also Pluto

And I see A1 was when they brought in the 3 MCB per RCD wasn't it

Was that part of the reason they brought in this rule in A1? Or was it more to do with limiting small amounts of earth leakage adding up over multiple circuits? Or a combination of both maybe?

Apr 02 2019 21:48

The rule of three MCBs on ONE RCD was to reduce the sum of earth leakage currents from 3 final circuit only on ONE RCD and to limit nuisence RCD tripping and also to reduce the effects on power loss on the electical installation if the only RCD tripped.

Apr 03 2019 07:07

Oh yeah, makes sense, thanks Pluto