Does a DNO 30mA RCD Isolator satisfy 411.4.204 (411.3.2.2)

Hello Gents, I have been out of the trade for a time (hence Im a bit rusty) and need to ask the following so I know where I stand regarding this scenario!

Does a DNO 30mA RCD Isolator satisfy 411.4.204 (411.3.2.2)

Scenario is: Domestic property, late 60's/early 70's, TN-S, old wylex 4 way fuseboard with recent wylex retrofit 60898 Type B MCB's fitted.

There is a 30mA RCD isolator present between the meter tails and consumer tails (rather than a 100A D.P switch).  (There is no other RCD present at the consumer unit or elesewhere.)

The EICR is due.  The R1+R2 and Zs on individual circuits have not been measured just yet.

Iam concerned that it might not be possible to meet max Zs times on the individual circuits.  In such a case, is it permitted to utilise the 30mA RCD mentioned above to provide requirements for fault protection (protection against electric shock) for the individual circuits?

My thinking is (from memory), if on a TT it is ok (usually theres no choice) to have some circuits on one side of a board protected by one 30mA RCD, then why would it not ok to do this on a TN-S / in the scenario above?

If the answer is No, what is the technical reason why not ok?

I do realise fitting a new consumer unit with RCBO's is probably the best solution, but this cant be done at this moment in time.

If the answer is yes it's ok, I have two more questions: 

1) When i read the schedule of test results, should I expect to see 1667 ohms written in the "maximum Zs permitted by BS7671" box/column for each circuit?

(assuming the MCB's dont meet max Zs)

and

2) Would item numbers 4.18 and 4.19 on the inspection schedule (page 481 in the regs) be ticked as 'acceptable condition'?

Thanks in advance, I will reply when I can.

John

On a TN system I always use the max Zs for the MCBs rather than the RCD/RCBO as this should easily be achieved on the vast majority of TN systems. The only time I use the RCD max is for TT systems.

I would comment, though not necessarily code, that the whole installation could be affected if there's a fault on one circuit.

Have you confirmed it is actually a TN-S supply?

I suspect that it may have been an TN-S with an isolator, then some work has been done and because it doesn’t comply as no RCD the spark swapped out the 100a switch for an RCD? 
what makes you think that it will not pass on ZS readings and ADS? 

Sharpend said:

Have you confirmed it is actually a TN-S supply?

I suspect that it may have been an TN-S with an isolator, then some work has been done and because it doesn’t comply as no RCD the spark swapped out the 100a switch for an RCD? 
what makes you think that it will not pass on ZS readings and ADS? 

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Its definitely TN-S. The RCD is of some age/has been in use for many years. Hopefully it will pass on Zs readings/ADS, but iam asking the question just in case it doesnt. I remember when I did my 2391 many years ago, we were taught to replace the MCB for a rcbo when you couldn't meet Zs, however as I explained I can't do that on this occasion. Thanks

Sidney said:

On a TN system I always use the max Zs for the MCBs rather than the RCD/RCBO as this should easily be achieved on the vast majority of TN systems. The only time I use the RCD max is for TT systems.

I would comment, though not necessarily code, that the whole installation could be affected if there's a fault on one circuit.

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Thanks for your reply. I agree with what you said. There must be the odd occasion (for whatever reason), that the MCB is swapped for a rcbo because that's what I was taught on the 2391 course. But I can't do that on this occasion, and that is why I was hoping someone could tell me if I can rely on the RCD protecting the board instead.

I seem to remember it being frowned upon as there is no reason to not design to comply with Zs value in most circuits, it is accepted on TT earthing as it is difficult to obtain a stable low Earth impedance. 
 

Reg: 411.4.5 suggests that an RCD maybe used for fault protection on a TN-S but not on a TN-C system. 

Regulation 411.4.5 suggests an rcd can be used for fault protection unless a TN-C system is in place as quoted above. In some circumstances your Ze may be high enough to warrant using an rcd on TN-S and TN-C-S systems where other protective devices cannot meet ADS. 

Sharpend said:

I seem to remember it being frowned upon as there is no reason to not design to comply with Zs value in most circuits, it is accepted on TT earthing as it is difficult to obtain a stable low Earth impedance. 
 

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It might have been frowned upon, but it was allowed. I guess if someone had not designed a circuit properly/extended it too far, or there was a bad connection somewhere that couldn't be found, then sticking an rcbo in would at least make it safe/get you out of hole. 

Back to the OP - what makes you think that the Zs won’t be satisfactory ?

sure if it was 3871’s but 60898?
 

The EICR will give the answers, if done properly by somebody with the necessary experience

Fleeting said:

Regulation 411.4.5 suggests an rcd can be used for fault protection unless a TN-C system is in place as quoted above. In some circumstances your Ze may be high enough to warrant using an rcd on TN-S and TN-C-S systems where other protective devices cannot meet ADS. 

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Thanks for that, indeed it does, but the regs/guides don't give a wide range of examples so I can't determine whether its OK for one RCD (protecting the board) to do the job or whether each potential problem individual circuit needs its own RCD (an rcbo). That is what iam asking. Would you say the answer to my main question is yes then? PS there is no TNCS involved it is TN-S. The local authority built and owned the houses upontil about 15 years ago I think, and assuming they did elec maintenance, then I would assume they would have installed a rod if necessary.

jbingham said:

It might have been frowned upon, but it was allowed. I guess if someone had not designed a circuit properly/extended it too far, or there was a bad connection somewhere that couldn't be found, then sticking an rcbo in would at least make it safe/get you out of hole. 

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Yes I don’t dispute that adding an RCD/RCBO wasn’t accepted as a remedial, I’m just saying that if it were new then you’d be expected to design accordingly.

Murdoch said:

Back to the OP - what makes you think that the Zs won’t be satisfactory ?

sure if it was 3871’s but 60898?
 

The EICR will give the answers, if done properly by somebody with the necessary experience

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Thanks, but Iam looking for a straight answer to my question(s). And I know the eicr will - IF it is done by somebody like you say.

Sure if type 2,3 BS3871's were in this scenario, then I would really struggle to meet Zs, but I haven't mentioned them!

Thanks for your replies so far chaps. Can anyone offer me definitive yes/no answer(s) to my question(s).

Thank you

jbingham said:

Thanks for your replies so far chaps. Can anyone offer me definitive yes/no answer(s) to my question(s).

Thank you

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Not sure I understand why the question is being asked ............

Murdoch said:

Not sure I understand why the question is being asked ............

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In a nutshell, is it OK for the 30mA RCD in the meter tails to provide shock protection for the final circuits - if the final circuits don't meet Zs (because the R1+R2 measures too high). And I'm asking this because I can't fit rcbo's. Plus there were 2 other questions at the bottom regarding forms.

Yes it is fine not an ideal situation but it shouldn't result in a unsatisfactory Report solely on the basis of this. 

jbingham said:

In a nutshell, is it OK for the 30mA RCD in the meter tails to provide shock protection for the final circuits - if the final circuits don't meet Zs (because the R1+R2 measures too high). And I'm asking this because I can't fit rcbo's. Plus there were 2 other questions at the bottom regarding forms.

Click to expand...

The word “if” means you don’t know what the readings are - or do you but you aren’t telling us

so, IMHO, if it’s a TNS or TNCS, and the Zs is outside of the readings for BS60898, I would then FI the circuit so it’s inspected for loose connections. Most people doing inspections have a good idea of the readings for types of usual homes

tin hat on

IMHO you are worrying about the wrong problem....

Unless this is a really really big property..

which I am guessing it isn't, (as only 4-way wylex box)..

I would be very worried if max Zs for some 60898's cannot be met on a domestic TN-s system?

e.g.  a 6A 60898, type B  Max Zs 7.28..   (table 41.3  pg 62)

so lets minus off 0.8 worth of possible Ze for a TN-S..

that gives us 6.48ohms worth of R1+R2 portion of the Max Zs..      (R1+R2=Zs-Ze)

So 6.48ohms is around 179m of 1.0mm/1.0mm T&E,    

Or  214m of 1.5mm/1.0mm T&E...

{on a lighting circuit}...  

150+ Meters is a BIG domestic lighting circuit!!!

How BIG is this house??

(Do the sums for 16A / 32A  2.5mm & 6.0mm)...?

If you cant get your Zs on a domestic TN-s,  then as Murdoch say's,  it needs FI..

Not a cowboy get out of jail 30ma RCD card!!!!

P.S.

How are you meeting 314.1 (i)  with just a single 30ma upfront RCD?

:C

Murdoch said:

The word “if” means you don’t know what the readings are - or do you but you aren’t telling us

so, IMHO, if it’s a TNS or TNCS, and the Zs is outside of the readings for BS60898, I would then FI the circuit so it’s inspected for loose connections. Most people doing inspections have a good idea of the readings for types of usual homes

tin hat on

Click to expand...

Agreed..

No need for tin hat IMHO!

Guinness

Concur with the honourable gentlemen. 
very well put Specs.