High Zs

[mcgaw81]

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.

 

[Andy™]

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?

 

[mcgaw81]

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.

 

[Andy™]

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.

9.8KW at what voltage?

or fit a different make 45A MCB

 

[mcgaw81]

9.8kw at 240v, Probably would get away with a 40amp?

 

[Andy™]

9.8kw at 240v, Probably would get away with a 40amp?

probably. its only a slight overload, but i doubt it would ever trip

 

[NozSpark]

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....

 

[mcgaw81]

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

 

[Andy™]

.46 seems high for 15mtr of 10mm twin

 

[NozSpark]

15m of 10.0mm T&E should have a (R1+R2) of 0.096ohms

so 0.44 is very high and you would expect the cable length to be just over 68m

 

[mcgaw81]

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

 

[mcgaw81]

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

 

[batty]

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

Mcgaw

What tester are you using.

Batty

 

[SPECIAL LOCATION]

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

 

[mcgaw81]

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

 

[batty]

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

 

[mcgaw81]

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

 

[SPECIAL LOCATION]

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

 

[fastham]

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

 

[mcgaw81]

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