Zs out of spec on an RCD protected circuit

[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?