Zs out of spec on an RCD protected circuit

[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