Zs out of spec on an RCD protected circuit

[sparksburnout]

Please accept my apologies if this is in the wrong category, and also my ignorance as I feel I should know this but I think the heat has confused me. In the theoretical scenario where you are doing an EICR on an RCD protected circuit, but the Zs for that circuit is actually (just, say) out of spec, how would you record it? And would you fail it?? Personally, I think it is obviously a fail, but with what code? Now, I am aware that the RCD is there for additional protection, and that it should not be relied on, and that the earth fault path should ensure the protective device does what it should, and obviously if there were no RCD it would be a code 2 (please correct me if wrong here). But is there sufficient justification to issue a code X on an RCD protected installation? I am referring to TN systems here, but does the 1667 max Zs for a TT have a bearing on this?

Tin hat out of cupboard......

 

[cprfenom]

Not having an RCD isn't always a code in the first place. It depends on the age of the installation and the method of installation. Personally although a circuit that has a Zs higher than the 1.15 would not be a fail straight off, you should still ask yourself why the readings are high and if you can bring the reading down by checking terminations etc. Your tests should be pointing you in the direction of what could be giving you a high ohms reading. I've always been told that codes are the opinion of the person doing the EICR and you are in the best place to determine the codes.

 

[sparksburnout]

Not having an RCD isn't always a code in the first place. It depends on the age of the installation and the method of installation. Personally although a circuit that has a Zs higher than the 1.15 would not be a fail straight off, you should still ask yourself why the readings are high and if you can bring the reading down by checking terminations etc. Your tests should be pointing you in the direction of what could be giving you a high ohms reading. I've always been told that codes are the opinion of the person doing the EICR and you are in the best place to determine the codes.

I get all that. Not having an RCD at all is no reason to fail anything as you say, provided it was installed when they were not a requirement. And I also get that it would be highly advisable to investigate why Zs is high, but, I was coming from the angle of failing it and recommending that remedial work may be required, bearing in mind the tester may not be the guy doing any remedial work. There could be many reasons for the Zs being high of course. My gut feeling would be to get it sorted and bring Zs into spec - but, what if some clever clogs customer/second opinion spark cited the fact that because it is RCD protected the circuit can't be failed?? Could you reasonable expect the customer to shell out to fix something that theoretically might not need fixing?? Again, personally, I know that I would recommend that it is sorted out every time, but what I was getting at is what code would you fail it with, if any???

 

[cprfenom]

If they are within permitted readings then I'm not sure I would give it a C2. I would be informing the customer that you are not happy with it and your reasons why. Apart from the readings have you found anything that would give you doubts about the integrity of the circuit? Grommets, earth sleeve, exposed conductors, junction boxes etc? The more you can add to your case the better.

I would just record the readings and put a note against it. Personally I would be telling the customer it needs further investigation to be able to give an honest report.

 

[Andy78]

Some food for thought... This has been lifted from the ESC best practice guide on EICR reporting and is used as an example of a C2.

• Earth fault loop impedance value greater than
that required for operation of the protective
device within the time prescribed in the version
of BS 7671/IET Wiring Regulations current at
the time of installation

A Zs value outside of the permitted values would simply not disconnect within the permitted time, therefore being potentially dangerous. The 1667 value would not have been relevant in all probability when the circuit was installed so has no bearing on your report.
I myself would be happy coding it as a C2 and would investigate if necessary and if instructed to. I wouldn't be going round trying to solve to fault if I had no prior arrangement with the customer though.

 

[sparksburnout]

Some food for thought... This has been lifted from the ESC best practice guide on EICR reporting and is used as an example of a C2.

• Earth fault loop impedance value greater than
that required for operation of the protective
device within the time prescribed in the version
of BS 7671/IET Wiring Regulations current at
the time of installation

A Zs value outside of the permitted values would simply not disconnect within the permitted time, therefore being potentially dangerous. The 1667 value would not have been relevant in all probability when the circuit was installed so has no bearing on your report.
I myself would be happy coding it as a C2 and would investigate if necessary and if instructed to. I wouldn't be going round trying to solve to fault if I had no prior arrangement with the customer though.

Exactly. I am coming at this from a purely hypothetical viewpoint, with no view to fixing anything, just reporting. If it was an install that was pre-RCD, then it's a fail C2, defo. But if there is an RCD, is it just a recommendation, ie a C3 then?? That is the point I am getting at. Or do we take it that the RCD is additional protection (not to be relied on?) and that it should be failed as a C2??? Personally, I am going down the road of a C3, but is this correct???

 

[sparksburnout]

I've just read my reply here, and I think it's a bit wrong. This is a C2 yes?? Irrespective of RCD or not?

 

[Andy78]

I would be very reluctant to code it a C3 just because it had additional protection via RCD.

C2 and further investigation required for me.....

 

[malcolmsanford]

The Zs Value for a 30ma RCD is 1667 and providing you obtain a reading below thls value on your testing then there is no code needed. The amendment makes it clear in the tables that if your RCD is provided for fault protection then 1657 is the value

 

[telectrix]

but, malcolm, if it was installed to s previous edition of the regs. where RCDs were not specified as being suitable for fault protection, surely it would be a C2?

 

[sparksburnout]

but, malcolm, if it was installed to s previous edition of the regs. where RCDs were not specified as being suitable for fault protection, surely it would be a C2?

ohh nice one Tel. Surely if the RCD is additional protection, it should be disregarded under these circumstances?? What if it fails, then you are back to 16th ed safety considerations.....??

 

[malcolmsanford]

Pretty much irrelevant because that is the fault protection device there now Tel. We can only test on what is the condition of the installation now, not why a protection device has been changed. Obviously if it was an RCBO and the cable was not sufficently sized for the overload side then you need to code it

 

[sparksburnout]

Pretty much irrelevant because that is the fault protection device there now Tel. We can only test on what is the condition of the installation now, not why a protection device has been changed. Obviously if it was an RCBO and the cable was not sufficently sized for the overload side then you need to code it

So in other words then, unless it exceeds 1667 ohms, the Zs figure is irrelevant if an RCD is protecting the circuit??? Can't be right surely??

 

[malcolmsanford]

Lets say your hypo question concerns an external lighting circuit connected in a ring main. Now some bright spark to win the contract works out that he needs a 6mm SWA and 250mts of it x price. But after calcs he can wire it in 4mm or even 2.5mm saving a fair bit but though VD is ok Zs on the cable is slightly to high what is the options open to him. Price for a larger cable and loose the contract or use a protective device that will give him his disconnect time for the cable size

 

[martinh287]

C3 installation is safe but requires investigation in why zs is higher then what is should have been before the rcd was installed

 

[sparksburnout]

Pretty much irrelevant because that is the fault protection device there now Tel. We can only test on what is the condition of the installation now, not why a protection device has been changed. Obviously if it was an RCBO and the cable was not sufficently sized for the overload side then you need to code it

Malc, your a well respected guy on here and I have no wish to cross swords with you! but, this can't be right? What is this high Zs reading is a portent of an underlying problem that might cause it to go beyond 1557 or whatever in the future?? Surely it should be recommended or even forced to be investigated, and so has to be a C2?? We can't just let an RCD be a cure-all surely?

 

[sparksburnout]

Lets say your hypo question concerns an external lighting circuit connected in a ring main. Now some bright spark to win the contract works out that he needs a 6mm SWA and 250mts of it x price. But after calcs he can wire it in 4mm or even 2.5mm saving a fair bit but though VD is ok Zs on the cable is slightly to high what is the options open to him. Price for a larger cable and loose the contract or use a protective device that will give him his disconnect time for the cable size

So you are saying that in these conditions, on a, say, 16th edition board, he would just use an RCD to protect that particular circuit and ignore the Zs value??

 

[sparksburnout]

C3 installation is safe but requires investigation in why zs is higher then what is should have been before the rcd was installed

Seems logical to me.....

 

[cprfenom]

That's what I was trying to get at before. If the readings were high and they put an RCD on, then its one way of bringing it in to line with regulations, but I would still be saying it needs further investigation. Its a similar argument to nan IR test and the 1 Mohm scenario

 

[sparksburnout]

Seems odd to me, the regs state an RCD is an additional protection, not a replacement protection, so how can you use the Zs value of the RCD if it is an addition? If you are going to use Zs based on the RCD, that is a replacement value surely??? Next we will be saying the IR value (to earth at least) doesn't matter either, cos the RCD will cover it?

 

[cprfenom]

What I mean by that is the reading for an IR should be out of range and not a few Mohm. In terms of your question I would ask this.

If you did do an EICR that had this situation and it didnt have an RCD, you would talk to the customer and explain to them the situation. Would you disregard putting in a RCD as a cost effective solution to a problem or would you insist on a circuit rewire and rcd?

 

[sparksburnout]

If I did an EICR as you and found this situation, then as it stands its a C2. Installing an RCD could arguably be masking a problem that should have every attempt made to remedy it. What if the high Zs value were caused by a deterioration somewhere that would get worse? If it was a 16th ed installation there is no requirement to install an RCD retrospectively as a work around for a problem in a particular circuit. Yes, I would always advise that an RCD is a very sensible measure, but it is not a cure-all and in my opinion should not be used to cover up actual problems

 

[sparksburnout]

What I mean by that is the reading for an IR should be out of range and not a few Mohm. In terms of your question I would ask this.

If you did do an EICR that had this situation and it didnt have an RCD, you would talk to the customer and explain to them the situation. Would you disregard putting in a RCD as a cost effective solution to a problem or would you insist on a circuit rewire and rcd?

is this not like failing an MOT for a seatbelt not working properly but just installing an airbag instead of fixing the seatbelt??

 

[malcolmsanford]

Guys guys please read the regs and especially 411.4.9 and then 411.5.3 and in neither of these regulations is the word additional used

Additional protection is used for sockets and outside equipment in reg 411.3.3

Guys I know where your coming from and all your scenarios are for the blatant variation of an expected value. IE on a domestic lighting circuit that would normally be 0.70-perhaps 1.20 ohm and you get a reading of say 6+ ohm. Then yes you should flag it up but a code 3 is improvement recommended and not further investigation so be careful what you put

As I said there are other reasons you use an RCD in relation to Zs values. An example again in a light domestic unit a company wins a contract and they need to install some temporary welding machines. The existing protection device is a B type breaker but a D type is required for the influx. You measure the Zs and it fails on a D type. So when the owner asks you what do I do as I will only be using the units for a month.

We all say well

A) You need to upgrade the circuit to a bigger cable to reduce the value. Costly and he is not going to like that

B) You need to reduce the MCB size or Type, not going to work

C) You fit an RCD to bring it into parameters.

For anyone that has done the 2391 course will know that you are often asked how to bring an Zs into line and those are the 3 options.

The trouble with all things electrical there is so many ways to skin a cat. There are times when you will have a higher Zs for he protection device installed originally and you have options to counter this, which I have outlined. The same goes with testing you have to assess and judge on the case in front of you, and not blanket, it is is o it has to be that

 

[sparksburnout]

Guys guys please read the regs and especially 411.4.9 and then 411.5.3 and in neither of these regulations is the word additional used

Additional protection is used for sockets and outside equipment in reg 411.3.3

Guys I know where your coming from and all your scenarios are for the blatant variation of an expected value. IE on a domestic lighting circuit that would normally be 0.70-perhaps 1.20 ohm and you get a reading of say 6+ ohm. Then yes you should flag it up but a code 3 is improvement recommended and not further investigation so be careful what you put

As I said there are other reasons you use an RCD in relation to Zs values. An example again in a light domestic unit a company wins a contract and they need to install some temporary welding machines. The existing protection device is a B type breaker but a D type is required for the influx. You measure the Zs and it fails on a D type. So when the owner asks you what do I do as I will only be using the units for a month.

We all say well

A) You need to upgrade the circuit to a bigger cable to reduce the value. Costly and he is not going to like that

B) You need to reduce the MCB size or Type, not going to work

C) You fit an RCD to bring it into parameters.

For anyone that has done the 2391 course will know that you are often asked how to bring an Zs into line and those are the 3 options.

The trouble with all things electrical there is so many ways to skin a cat. There are times when you will have a higher Zs for he protection device installed originally and you have options to counter this, which I have outlined. The same goes with testing you have to assess and judge on the case in front of you, and not blanket, it is is o it has to be that

What about 415.1.1 and 415.1.2 then??

 

[Richard Burns]

What about 415.1.1 and 415.1.2 then??

415.1.2 specifies that if an RCD is used then the protective measures of ADS, SELV, Double insulation, or electrical separation must be used.
For ADS compliance, if the RCD meets the disconnection time AND all the other requirements for ADS are in place (including short circuit and overcurrent protection) then the RCD can be used as the means of disconnecting the circuit in the case of an earth fault.

Regarding the Zs out of spec for an MCB but in spec for an RCD situation you would need to consider if there was a fault on the circuit causing the high Zs, or if the circuit was designed that way.

If the test results do not come close to meeting the values you would expect from a theoretical assessment of the circuit then you may well consider that there is a loose connection or some such that could therefore be dangerous in the case of a fault and so code as a C2.

If the test results match with what you would expect from the circuit and so you believe there are no faults in the circuit, then the circuit is safe for use and will not present a risk and would therefore not be coded.

 

[sparksburnout]

415.1.2 specifies that if an RCD is used then the protective measures of ADS, SELV, Double insulation, or electrical separation must be used.
For ADS compliance, if the RCD meets the disconnection time AND all the other requirements for ADS are in place (including short circuit and overcurrent protection) then the RCD can be used as the means of disconnecting the circuit in the case of an earth fault.

Regarding the Zs out of spec for an MCB but in spec for an RCD situation you would need to consider if there was a fault on the circuit causing the high Zs, or if the circuit was designed that way.

If the test results do not come close to meeting the values you would expect from a theoretical assessment of the circuit then you may well consider that there is a loose connection or some such that could therefore be dangerous in the case of a fault and so code as a C2.

If the test results match with what you would expect from the circuit and so you believe there are no faults in the circuit, then the circuit is safe for use and will not present a risk and would therefore not be coded.

Brilliant Richard I think I am pretty clear on this now