Plugging an RCD into a cct already protected by an RCD/RCBO

[Sparky54]

Pardon my lack of electrical knowledge and ignore the Sparky user name but I was wondering if an RCD plugged into a ring main powered via an RCBO would not be as effective due to them sharing the trip current, or will the most sensitive of the pair still trip first at ~30mA ?

 

[JohnW2]

Pardon my lack of electrical knowledge and ignore the Sparky user name but I was wondering if an RCD plugged into a ring main powered via an RCBO would not be as effective due to them sharing the trip current, or will the most sensitive of the pair still trip first at ~30mA ?

It would't be any less effective (exactly the same current inevitably flows through both devices), but it would be a matter of chance as to whether one, the other or both tripped in the case of a fault.

Most people would probably say that it's unnecessary to plug in an RCD if the circuit is already protected by another RCD or RCBO. The only situation in which the second device would offer any advantage is if the first one was faulty and failed to trip when it should.

Kind Regards, John.

 

[Paul_C]

An upstream RCD won't in any way desensitize the one downstream of it, since the same currents (on each side of the circuit) are passing through both (assuming that the fault isn't between them, of course).

However, you can't guarantee that one and only one will trip on an earth fault. Due to the tolerances of the actual current required to trip a particular breaker and its response time at that current, you can find that one or the other or both will trip on a given fault. Even if the upstream unit is, say, 100mA and the downstream RCD is 30mA there's no guarantee that the 30mA device will trip first if the fault current is high enough to also cause the 100mA RCD to trip.

That's why with cascaded RCD's it's common for the upstream one to be a time-delayed unit, to try avoid the inconvenience of a greater part of the installation losing power than is necessary to isolate the fault.

 

[holmslaw]

..

 

[ban-all-sheds]

ignore the Sparky user name

No.

You deliberately chose it despite being fully aware of your lack of electrical knowledge.

Why?

 

[holmslaw]

..

 

[JohnW2]

.... Even if the upstream unit is, say, 100mA and the downstream RCD is 30mA there's no guarantee that the 30mA device will trip first if the fault current is high enough to also cause the 100mA RCD to trip. ... That's why with cascaded RCD's it's common for the upstream one to be a time-delayed unit ....

I've still got 100mA Type S RCDs at the origin of my (TT) installation - basically a leftover from the days before all circuits were RCD/RCBO protected, but retained as useful for isolation (as well as 'belt and braces' RCD protection).

I've often wondered why the traditional practice seemed to be to use 100mA, as well as time-delayed, RCDs in that situation. As you say, the 30mA/100mA difference by no means ensures discrimination, that being achieved by the time delay. So why didn't they use time-delayed 30mA RCDs? Was it perhaps because leakage currents in whole installations were often above 30mA in those days?

Kind Regards, John.

 

[ban-all-sheds]

Whats it got to do with you, mind your own business.

It is my business, as it is that of any responsible person.

What if at some point Sparky54 incorrectly thinks he knows the answer to somebody else's question and answers it incorrectly?

Choosing a user name like that carries with it the implication that the user is an electrician, and that anything he says should be taken as if being said by an electrician. Possibly one who is 54 or 57 years old.

Sparky54, ignore BAS he thinks he owns this place and loves to bully new members.

I think you'd have to be particularly feeble and pathetic to feel hurt, persecuted or intimidated by

No.

You deliberately chose it despite being fully aware of your lack of electrical knowledge.

Why?

 

[Paul_C]

I've often wondered why the traditional practice seemed to be to use 100mA, as well as time-delayed, RCDs in that situation. As you say, the 30mA/100mA difference by no means ensures discrimination, that being achieved by the time delay. So why didn't they use time-delayed 30mA RCDs? Was it perhaps because leakage currents in whole installations were often above 30mA in those days?

No more likely to be so than today, I would say, as "those days" aren't really all that long ago.

I can't think of anything I've seen in print regarding this, but my inclination would be to think it's probably just because 100mA was the standard for all regular RCD protection (typically whole-house) immediately before 30mA became common.

 

[JohnW2]

... Was it perhaps because leakage currents in whole installations were often above 30mA in those days?

No more likely to be so than today, I would say, as "those days" aren't really all that long ago.

Continuation of the practice certainly wasn't long ago - indeed, I'm sure that it continues to today. However, it was probably a fair while ago that the fitting of these RCDs started, and hence the 100mA habit got established, wasn't it?

I can't think of anything I've seen in print regarding this, but my inclination would be to think it's probably just because 100mA was the standard for all regular RCD protection (typically whole-house) immediately before 30mA became common.

Yes, that might well be the case. The 'voltage-operated' (somewhat of a misnomer!) devices which preceded them were also often/generally 100mA, weren't they?

Kind Regards, John.

 

[Sparky54]

Thanks for all the excellent info gents. I wasn't sure how the leakage current was detected and wondered if the two devices might somehow share it.

Most helpful and much appreciated.

Sparky54

 

[Electrifying]

30 mA RCDs are for additional protection against electric shock - so you wouldn't have one up-front on a TT installation.

The 100mA is for 'earth fault protection' because the Ze won't be low enough to meet disconnection times on an MCB.

Now, what I'm worried about here, is that John is going to go into one of his.......'well, the 30mA is also offering earth fault protection blah blah blah'
We know this, John, but, like the 'Earthing/Bonding' - dual function discussion...........I'm just telling it as it is

'Additional Protection' from a 30mA RCDs may not be required on all circuits of an installation - but 'earth fault' protection is........so, on a TT, the chances are you need an RCD up-front.

Also, with them being for 'earth fault protection', you can fit a size RCD to suit your Ze/Ra........so it doesn't necessarily have to be 100mA either.

 

[JohnW2]

30 mA RCDs are for additional protection against electric shock - so you wouldn't have one up-front on a TT installation.
The 100mA is for 'earth fault protection' because the Ze won't be low enough to meet disconnection times on an MCB.

No argument with any of that, except that I see no reason (unless the installation's total leakage currents are too high) why the 'up-front' RCD providing earth fault protection could not be a 30mA (Type S) one.

'Additional Protection' from a 30mA RCDs may not be required on all circuits of an installation - but 'earth fault' protection is........so, on a TT, the chances are you need an RCD up-front.

That is obviously much more likley to be the case with pre-17th ed. instals, notably 'split load' ones, but a lot of installations under current regs will have all circuits having ('additional') RCD/RCBO protection, and therefore technically do not require an additional 'up front' device (if they are TT) to provide earth fault protection.

As I think I said before, my installation is like that. All of the circuits in my (TT) installation have RCD or RCBO protection ('additional', if you must) and therefore I don't actually need the 'up front' 100mA Type S RCDs - but I've kept them as isolators and as belt-and-braces ('additional additional', if you wish).

Also, with them being for 'earth fault protection', you can fit a size RCD to suit your Ze/Ra........so it doesn't necessarily have to be 100mA either.

At last! That was actually the one and only point I was making/asking - why it is that people came to use 100mA, rather than 30mA (time-delayed) onesfor this purpose. Do you know? Was it because total installation leakage was commonly too high to use 30mA ones, because most RCDs were 100mA when the habit was established, or what?

Kind Regards,John.

 

[Electrifying]

John, I tend to think that the 30mA that we require for 'Additional Protection' would be deemed too sensitive to be the RCD of choice for 'earth fault protection'.
Don't forget, the regulation (can't remember no. or location) that applies to a fault on one circuit not affecting other circuits etc etc. has to have come from somewhere......and this is probably it.

It doesn't take much to trip a 30mA RCD (which is the whole point), so I don't think you'd want one covering the whole installation.

This, though, John, I can't agree with.

That is obviously much more likley to be the case with pre-17th ed. instals, notably 'split load' ones, but a lot of installations under current regs will have all circuits having ('additional') RCD/RCBO protection, and therefore technically do not require an additional 'up front' device (if they are TT) to provide earth fault protection.

As I think I said before, my installation is like that. All of the circuits in my (TT) installation have RCD or RCBO protection ('additional', if you must) and therefore I don't actually need the 'up front' 100mA Type S RCDs - but I've kept them as isolators and as belt-and-braces ('additional additional', if you wish).

There are numerous reasons why a customer might require a socket or other circuit without 30 mA RCD protection - and numerous ways to still comply with the Regs - but, if they're on a TT system, they would still require an RCD for 'earth fault protection'.

 

[JohnW2]

John, I tend to think that the 30mA that we require for 'Additional Protection' would be deemed too sensitive to be the RCD of choice for 'earth fault protection'. Don't forget, the regulation (can't remember no. or location) that applies to a fault on one circuit not affecting other circuits etc etc. has to have come from somewhere......and this is probably it.

Yes, that's a good point, now. However, I'm not at all sure that the concept of division/separation of circuits (I presume you mean 314 of current regs) will actually have existed at the time the '100mA habit' was established (probably 15th ed), Indeed....

It doesn't take much to trip a 30mA RCD (which is the whole point), so I don't think you'd want one covering the whole installation.

Agreed - but you are again talking about now/current regs,not the time at which the '100mA habit' was established. In earlier times, it was quite common to have a whole installation getting 'additional protection' from a single 30mA covering all circuits (with all earthing systems). I certainly had such a system at one point in time, and some members of my family still do.'

This, though, John, I can't agree with.

...a lot of installations under current regs will have all circuits having ('additional') RCD/RCBO protection...

There are numerous reasons why a customer might require a socket or other circuit without 30 mA RCD protection - and numerous ways to still comply with the Regs - but, if they're on a TT system, they would still require an RCD for 'earth fault protection'.

I don't disagree with any of that, but think this is reducing to quibbling again. All I said was "a lot of installations under current regs...", and I still think that's true (mine being one example). Of course there are a good few which have some circuits without RCD protection, and if they are TT, then some provision for earth fault protrection will be required - but there are nevertheless many/'a lot'/whatever for which that won't be the case. I really don't think there is much point in arguing about what consitutes "a lot of", do you?

Kind Regards, John.