RCD Testing - Which Regulations?

[imroberts]

Apologies in advance for the lengthy post, and many thanks to those who make it to the end and provide me with an answer

While studying for my 2382-12 exam, I encountered the following question:

When testing a 100mA RCD the maximum test current applied via the test instrument will be

A. 3 x In
B. 5 x In
C. 1 x In
D. 1.5 x In

I believe the correct answer is C?

My understanding of the correct test procedure is this:

1) Test at 0.5 x In - RCD shouldn't trip
2) Test at 1 x In - RCD should trip within 200ms
3) Test at 5 x In - RCD should trip within 40ms
4) Check operation via Test button

Upon researching this question further, I find that Guidance Note 3 suggests the 5 x In test is only necessary on RCDs upto 30mA. However I'm struggling to find reference to this in the regulations - I see that 415.1 states that where a 30mA RCD is used for additional protection, it should trip within 40ms at 5 x In - does the fact this isn't stated for other scenarios or other uses of RCDs mean that those other scenarios don't require such testing? (Me being me is likely to test all RCDs as such anyway just for completeness and peace of mind)

Secondly, where in the regulations is it stipulated (or implied or calculated) that the RCD shouldn't trip at 0.5 x In, and that this should be tested?

 

[JohnW2]

I see that 415.1 states that where a 30mA RCD is used for additional protection, it should trip within 40ms at 5 x In - does the fact this isn't stated for other scenarios or other uses of RCDs mean that those other scenarios don't require such testing?

What 'other scenarios or uses' did you have in mind?

Kin Regards, John.

 

[imroberts]

I see that 415.1 states that where a 30mA RCD is used for additional protection, it should trip within 40ms at 5 x In - does the fact this isn't stated for other scenarios or other uses of RCDs mean that those other scenarios don't require such testing?

What 'other scenarios or uses' did you have in mind?

Kin Regards, John.

I was thinking of scenarios such as socket outlets on a caravan park, which are stipulated to be RCD protected. I just looked up in the regs though and these all seem to give reference to 415.1.1 anyway which means they *do* fall under the above

How about a scenario where a 300mA RCD would be fitted for protection against fire? I'm guessing this would only require testing at 1 x In?

 

[RF Lighting]

Presumably scenarios such as when the RCD is used to provide fault protection on a TT installation for example.

 

[JohnW2]

Presumably scenarios such as when the RCD is used to provide fault protection on a TT installation for example.

Particularly in view of other ongoing threads, I was expecting that one. However, in that scenario, I think it's fair to say that the RCD is providing both primary fault protection (i.e. in the event of a negligible impedance L-E fault) and also 'additional protection' (disconnecting in response to a small {30mA or whatever} L-E 'leakage', perhaps through a human being). Am I wrong?

Kind Regards, John.

 

[Electrifying]

Hi,

The correct answer is C

Your RCD test procedure should include 180 degree test for 1xIn and 5xIn.

RCDs used for earth fault protection are required to disconnect in 0.4s at 230V (TN systems), so there is no need to test them at 5xIn if 1xIn will disconnect in 0.2s. - does that make sense?

30mA RCDs used for 'additional protection' are required to disconnect in 0.04s (40ms) at <50V, so to ensure this will occur we test at 5 times the operating current (150mA) at 230V.

The RCDs are tested at 1/2xIn to ensure they won't trip on slight earth leakages, which would give rise to nuicance tripping.

This is purely my understanding of it.......others may have there own reasonings.

 

[RF Lighting]

Presumably scenarios such as when the RCD is used to provide fault protection on a TT installation for example.

Particularly in view of other ongoing threads, I was expecting that one. However, in that scenario, I think it's fair to say that the RCD is providing both primary fault protection (i.e. in the event of a negligible impedance L-E fault) and also 'additional protection' (disconnecting in response to a small {30mA or whatever} L-E 'leakage', perhaps through a human being). Am I wrong?

Kind Regards, John.

I was thinking more of your 100mA TD up front RCD which were common on 16th edition TT installations.

 

[JohnW2]

I was thinking of scenarios such as socket outlets on a caravan park, which are stipulated to be RCD protected. I just looked up in the regs though and these all seem to give reference to 415.1.1 anyway which means they *do* fall under the above

That's what I thought

How about a scenario where a 300mA RCD would be fitted for protection against fire? I'm guessing this would only require testing at 1 x In?

Doesn't that still count as 'additional protection'? I'm really not sure. edit: I think that I've answered my own question below. It seems that, per definition of 'additional protection' in 415.1.1, it can't be regarded as 'additional protection' if I&#916;n >30mA

Kind Regards, John.

 

[JohnW2]

I was thinking more of your 100mA TD up front RCD which were common on 16th edition TT installations.

(I still have some) ... and in 17th edition TT installations if there are any circuits which don't have (and aren't required to have) RCD protection.

But, yes, I think you're right. On reflection, the definition of 'additional protection' in 415.1.1 (there is no definition in Part 2) is actually such that it can only apply to RCDs with I&#916;n no greater than 30mA.

Kind Regards, John.

 

[imroberts]

RCDs used for earth fault protection are required to disconnect in 0.4s at 230V (TN systems), so there is no need to test them at 5xIn if 1xIn will disconnect in 0.2s. - does that make sense?

Yes, makes perfect sense. No point testing for a requirement we've already exceeded

The RCDs are tested at 1/2xIn to ensure they won't trip on slight earth leakages, which would give rise to nuisance tripping.

Is this purely a "best practice" rather than a strict requirement of the regulations?

 

[Electrifying]

The RCDs are tested at 1/2xIn to ensure they won't trip on slight earth leakages, which would give rise to nuisance tripping.

Is this purely a "best practice" rather than a strict requirement of the regulations?

The requirement is to avoid unwanted tripping due to excessive protective conductor currents - so I suppose ensuring the RCD won't trip at less than half its rated current goes some way to prevent this.
It's no good if it's going to trip at 2mA.

 

[EFLImpudence]

Is this purely a "best practice" rather than a strict requirement of the regulations?

Not exactly, the manufacturing requirement is that the RCD must trip between 15mA and 30mA
Usually they trip at 25mA to 29mA.

So, testing that it does not at 15mA and does at 30mA ensures that it is correct.

 

[JohnW2]

The RCDs are tested at 1/2xIn to ensure they won't trip on slight earth leakages, which would give rise to nuisance tripping.

Is this purely a "best practice" rather than a strict requirement of the regulations?

Excessive nuisance tripping could probably be regarded as being non-compliant with 132.1(ii) or one of the other 'catch all' sections of the regs.

Kind Regards, John.