help in understanding RCD ratings

mmax6870 · Jul 29, 2009

Hi All,

I am due to start year 2 of C&G2330 L2 in September and wish to get my head around RCD ratings. I think I have grasped the principle of how they work eg: ensuring the line and neutral currents balance. However, I am not sure about the ratings printed on the units themselves.

Looking through numerous catalogues, I see that they have 2 disconnection related figures quoted eg: 63A 30mA or 40A 30mA but I am not sure what these figure relates to.

If anyone can assist I would be most grateful.

Mark

Darkwood · Jul 29, 2009

If you are talking about rcd's only then the mA rating is what leakage to earth will cause it to trip so a 30mA rcd will trip if their is a earth fault with a leakage current of 30mA or more (in reality they trip between 50% and 100% of this rating), the amp rating is what load you can run through the rcd unit so a 40amp 30mA rcd can safely carry up to 40amps if you exceed this you could burn the rcd unit out making it inoperative but usually you have a mcb or fuse in line limiting this from happening.
Note rcbo's are the same for the mA ratings but the ampage is their rated overload protection value as they are a rcd and mcb combined into one unit.

Risteard · Jul 29, 2009

darkwood said:
(in reality they trip between 50% and 100% of this rating)

In fairness it shouldn't trip at 50% of the rated operating current, as the *1/2 test on should not result in the RCD tripping. This helps prevent nuisance tripping.

Darkwood · Jul 29, 2009

Risteard said:
In fairness it shouldn't trip at 50% of the rated operating current, as the *1/2 test on should not result in the RCD tripping. This helps prevent nuisance tripping.

Fair point but its rare anyway its just what the manufactures state in their technical info, i think they are just giving themselves a wider birth but in practice ive never tripped one at 0.5 test anyway.
Although i did say between 50% and 100% and not 50% on the nose and meters are quite accurate at delivering an exact mA when needed but get your point.

mmax6870 · Jul 29, 2009

darkwood said:
If you are talking about rcd's only then the mA rating is what leakage to earth will cause it to trip so a 30mA rcd will trip if their is a earth fault with a leakage current of 30mA or more (in reality they trip between 50% and 100% of this rating), the amp rating is what load you can run through the rcd unit so a 40amp 30mA rcd can safely carry up to 40amps if you exceed this you could burn the rcd unit out making it inoperative but usually you have a mcb or fuse in line limiting this from happening.
Note rcbo's are the same for the mA ratings but the ampage is their rated overload protection value as they are a rcd and mcb combined into one unit.

Thanks Darkwood, that makes sense. Does this mean then that all mcb's in line should not be over 40amps in total?

Darkwood · Jul 29, 2009

mmax6870 said:
Thanks Darkwood, that makes sense. Does this mean then that all mcb's in line should not be over 40amps in total?

If the rcd covers just one mcb then yes but if it covers more than one mcb like a front end rcd 100amp 30mA then the loading has to be worked out with diversity in mind just to make sure your rcd isnt damaged through normal use.

mmax6870 · Jul 29, 2009

darkwood said:
If the rcd covers just one mcb then yes but if it covers more than one mcb like a front end rcd 100amp 30mA then the loading has to be worked out with diversity in mind just to make sure your rcd isnt damaged through normal use.

I assume the diversity factors are given in the red book or somewhere else perhaps?

Darkwood · Jul 29, 2009

mmax6870 said:
I assume the diversity factors are given in the red book or somewhere else perhaps?

Il let someone else answer that one for you its a long time since i used them as im on the industrial machinery now, goodluck anyway.

mmax6870 · Jul 29, 2009

darkwood said:
Il let someone else answer that one for you its a long time since i used them as im on the industrial machinery now, goodluck anyway.

Thanks for your help

piz · Jul 29, 2009

Diversity is on Page 97 of the on site guide hope that helps

mmax6870 · Aug 13, 2009

Thanks for that Piz, been away on holiday, so just picked up message.

Mark

hidex1213 · Nov 11, 2011

geeez I work in an electrical supplies company and believe or not, even a lot of our senior colleagues have confusions about this. Lots of our customers (electricians with years of experience), and even the appointed distributor of Mitsubishi failed to answer this question. I thought that the AMP rating on an RCD should only states its ability to withstand a certain amount of current, until not only one person refused to take a 40A RCD instead of a 32A one for the same price...

Problem with this industry here in Hong Kong is that people either have very good education in this field or they have virtually none. And those who're very educated wouldn't not be too much involved with us....sigh*

Engineer54 · Nov 11, 2011

hidex1213 said:
geeez I work in an electrical supplies company and believe or not, even a lot of our senior colleagues have confusions about this. Lots of our customers (electricians with years of experience), and even the appointed distributor of Mitsubishi failed to answer this question. I thought that the AMP rating on an RCD should only states its ability to withstand a certain amount of current, until not only one person refused to take a 40A RCD instead of a 32A one for the same price...

Problem with this industry here in Hong Kong is that people either have very good education in this field or they have virtually none. And those who're very educated wouldn't not be too much involved with us....sigh*

I don't think i've ever seen an RCD with a 32A rating, normally 40, 63, 80, 100. Are you sure your not getting RCD confused with RCBO??

And as darkwood posted earlier Those current ratings on RCDs are the current ratings that the RCD has been designed to safely carry. The mA rating being the only protective module (trip) within the unit. RCBOs on the other hand are a combined MCB/RCD and therefore Both current (A) and mA ratings refer to the protection levels (trips) incorporated within the unit

telectrix · Nov 11, 2011

I don't think i've ever seen an RCD with a 32A rating, normally 40, 63, 80, 100. Are you sure your not getting RCD confused with RCBO??

they are made. fitted quite a few in factory floor situation where there are maybe 2 or 3 socket outlets at a work station, each work station having it's own circuit from a DB with the RCD fitted at the work station. ( 32A/30mA )

hidex1213 · Nov 12, 2011

Yes I'm sure it's an RCD instead of an RCBO. ie. it won't trip in case of an overload. Mitsubishi's making them 32A ones - for the same price of a 40A; which is exactly one big reason why I was so confused! Who would choose one with simply less load capacity for the same price.? But you're right, most of them make 25A 40A and 63A. By the way, who's making those 100A RCDs these days? (NOT the ones with gigantic size. I want one that has the size of a regular 4P MCB only.) Schupa seems to have them but those cost a fortune! Got alternatives...?

friggin'...
remember I mentioned asking the dealer of Mitsubishi power distribution products about my confusion?
I was like, "so uh...I suppose an RCD won't trip on an overload, so why are you selling 40A and 32A ones for the same price? What does that 40 and 32 mean?"
Her, "It's the amp!"
me, "Well I know, but what does that indicate? Is it where it would trip or what?"
Her, "It's the amp rating!"
me, "......"

hidex1213 · Nov 12, 2011

Engineer54 said:
until not only one person refused to take a 40A RCD instead of a 32A one for the same price...

You read it just right.
Most of them would tell me things like "cuz the MCB is 32A only and they gotta match" or "oh I don't want the amp rating to be so high"...
when I asked "why?", they'd all seem to get tipped off and refuse to continue the conversation.
NOW I think they simply don't know what they're saying...