High Zs

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mcgaw81

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I wired a shower last week and went back to connect it up and test today, however on testing the Zs of the circuit i get a reading of 0.93ohms, and the MCB maximum is 0.92ohms (50amp Type B) in regs, on the back of my NICEIC test books its 80% which allows 0.73ohms.

More detail, what I have is a Sub Main supplied in 16mm Twin and Earth from a 60amp switch fuse. I have install new 4 way enclosure with 63amp RCD 50amp MCB (shower) 16amp (1x Double skt).

At the top of the RCD i get 0.22ohms, socket is 0.44ohms and the high reading for the shower. Everything else tests ok, my R1+R2 was 0.46ohms. Now i know i could have done my Zs by calculation and would have had a satisfactory result but why is my R1+R2 ok but high Zs.

What are my options of reducing this, different Type MCB, maybe type 1? What other options do I have?

Thanks in advance.

 
try with another test kit?

These days, only type B,C and D are available.

could a 40 or 45A MCB be used instead?

or run another earth along cable?

or re connect all terminals, make sure they are tight and not adding to resistance?

 
It was a cheap proteus board andy, and they only have 40 or 50amp. Its a 9.8kw shower hence why i went for 50amp.

 
Firstly you are measuring your Zs with Trip loc so you will get slightly higher readings!!

also your RCD and switch contacts could have a slightly higher resistance than you would expect. Operate them a few times to clean the contacts? :| ?:|

What cable have you run to the shower and how long is it, cause from your (R1+R2) you seem to have quite a long run....

 
Its 10mm twin and earth, the run turned out to be longer than expected, about 15m.

 
I thought it was very high, could I have a loose earth in switch or damaged cable. Never noticed anything thou???

 
I'll test it with another tester could be that, although my tester was only calibrated at start of feb

 
Its 10mm twin and earth, the run turned out to be longer than expected, about 15m.
sounds like a loose connection somewhere there m8?

is there a cord pull isolator switch?

IF your R1+R2 0.46 is on 10mm with 9800watt rated shower @ 240v

then this would be outside max permissible Volt drop as well!

0.46 of 10mm is 71m

(from pg 166 OSG 10.0mm = 6.44mohm/m (0.46/6.44)x1000=71.4m)

which if you only put in 15m? this is way out!!!

Volt drop on 10mm is 4.4mohm/A/m

max permissible is 11.5v (5% NON lighting circuit)

9800w/240v = 40.8amps

so 11.5 = (4.4 x 40 x length)/1000

and 11.5 = 0.176 x Length

Length = 11.5/0.176 = 65.3m

65.3m of 10mm would have an R1+R2 of 0.42.

So your existing R1+R2 is too high for volt drop.

Also as Noz says..

if only 15m of 10mm you should be getting an R1+R2 of 0.0966

personally allowing for the odd poor connection with your meter leads..

and maybe a screw terminal or switch contact...

I'd be happy with something looking around 0.10ohm - 0.15ohm

 
Thanks for the input lads, going back next saturday to have a look, can't get earlier as the guy just bought the house and stays down south. Will report back!.

Batty, I have a megger mft1502

 
Thanks for the input lads, going back next saturday to have a look, can't get earlier as the guy just bought the house and stays down south. Will report back!. Batty, I have a megger mft1502
Was wondering if it was a fluke. My fluke has been playing up i am hoping its the leads but I think it maybe something else because the other day the display kept going and then it made some funny noises and cut out. I was getting some strange readings also.

Batty

 
As i said earlier had it calibrated start of Feb by 'mike' and purchased a new set of leads at the same time. I will just have to wait till next week and do some investigating

 
As i said earlier had it calibrated start of Feb by 'mike' and purchased a new set of leads at the same time. I will just have to wait till next week and do some investigating
Have you got a check box or Calcard?

these are the times they come in very useful..

verify your meter & leads are all ok

I can highly recommend the Caltronics Calcard!Applaud Smiley:)

 
HI guys,

Aside from the way out reading of r1 +r2 on 10mm,

can someone confirm that if you have correct mcb and also protection by an rcd the max zs allowed is 1667 taken from table 41.5 page 50 17th edition.

thanks

 
Bought my Calcard on wednesday, probably arrive tomorrow so will get to check the tester.

 
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