Ze Reading query

[telectrix]

Hi, peeps, just installed a small ring in a farm barn, R1+R2 =0.02ohms, Zs at mid point = 0.64ohms. when i measured Ze, i got 0.67ohms. now, unable to dis. main earth as it's a sub-main, fed in SWA using armour as the earth. with parallel paths, i would have expected the error to be on the lower side rather than the upper. as readings are within limits, should i investigate further or leave well alone?

 

[malcolmsanford]

I'm assumming that it's a normal ring final circuit with BS 1363 sockets and plugs. so then as per reg 705.411.1 it will be RCD protected with a 30mA. then Zs will be 1667.

So it's a leave well alone

 

[IQ Electrical]

Would you not have expected 0.69 and measured 0.64?

I'd be quite happy with that, remember, loop impedance testers are rather inaccurate at values of less than 1 Ohm.

I've got a paper on the inaccuracy and how to get round it by incorporating a known value (an R2 lead etc.) into the circuit under test.

I'll see if I can dig it out!

 

[telectrix]

the ring consists of one 1363, twin socket, and a 16A round pin industrial socket . fed from 32A BS88 in old RedSpot Board, via 40A,30mA RCD.

 

[Guest123]

I think I'd be a bit more worried about a 16A rated socket outlet protected by a 32A fuse. The socket & plug arrangement would melt before any overload protection operated if someone unknowingly plugged a larger load in.

 

[telectrix]

good point, malcolm. can easily reduce Bs88 to 20A as the 1363 is only for occassional power tools

 

[malcolmsanford]

Nearly always good points old mate lol

I have to admit it's not the way I would have done it, having the chance of an unfused 16amp plug conecting into that 60309-1/2 with a bigger than a 16amp load could be worrying as the BS 88 is 32amp. You could get the situation of the acessory geting a tad warm.

Also not sure why you fitted a 40amp RCD with that 32amp BS 88, was it the fact that you had a 40amp knocking about? Would have personally kept the In the same for both protection devices.

Must admit I would have come out of the 32amp BS 88 to a little CU with a 32amp 30mA RCD and then a couple of 16amp MCB for the each sockets. I know your getting overlaod protection with the BS 88, but I would have given each socket it's own overload for discrimination.

 

[telectrix]

40A RCD was the nearest they had in stock. beraing in mind that 40A is the max. rating of the RCD, orig. spec was for twin 13Asocket only, in 20mm pvc tube. 16A socket was afterthought, so i just ringed it as is adjacent to 13A. thinking was 32Afuse to protect ring 2.5mm, TBH< never thought about poss. overload on 16A socket. DOH.

 

[telectrix]

I think I'd be a bit more worried about a 16A rated socket outlet protected by a 32A fuse. The socket & plug arrangement would melt before any overload protection operated if someone unknowingly plugged a larger load in.

thinking about this... anything that could be plugged in would be rated at a max. of 16A and have a 16A plug fitted. even if it's on a farm, i can't see anything overloading the socket unless it's faulty/ short circiut, then it will take out the 32.

 

[Guest123]

This is a farm.....with farmers, it would take nothing for them to put a samller plug on their 30A welder to get it to work.......for a while at least.

I've seen nails in place of fuses, 22mm copper pipe in place of service fuses etc. I do a lot of farm work living where I do and some of the situations I come across are proper scary.

 

[telectrix]

true, but this farmer is fairly well clued up. that's why he's got me in to make sure everything is safe. having said that, i think it might be an idea to downrate the 88 to 20A. what do you think?

 

[Jimmy Boy]

I have probably got the wrong end of the stick with the OP's comments but can someone clarify, I have always been taught that Ze must be taken at the "Origin" of the installation not sub mains or dis boards, this is the only way to measure it accurately, assuming you don't want to do it by enquiry.

 

[IQ Electrical]

Yes, there is only one Ze for an installation, at the origin with the earthing conductor removed from potential parallel paths.

 

[telectrix]

have to enquire as disconnecting earth not practical whilst harvesting with machinery running. or measure it at a later date.

 

[Jimmy Boy]

Hi, peeps, just installed a small ring in a farm barn, R1+R2 =0.02ohms, Zs at mid point = 0.64ohms. when i measured Ze, i got 0.67ohms. now, unable to dis. main earth as it's a sub-main, fed in SWA using armour as the earth. with parallel paths, i would have expected the error to be on the lower side rather than the upper. as readings are within limits, should i investigate further or leave well alone?

Sorry fella I may have got wrong end of the stick, your post read as you had taken a Ze at sub main or dis board, this is contrary to regs and seems pointless, as the values you measure are on parallel paths, I am happy to be corrected on regs but I believe your not supposed to energise a circuit until you have established main earth to the supplier, even if you do Ze by enquiry.If I am wrong I am happy to be chastised.

ATB S