Newly Fitted 17th Edition Consumer Unit

[KENRUCK1956]

Hi,
I have just fitted a New consumer unit as my accessment job to get registered.
The layout is:
100amp isolation switch
6 amp RCBO for smoke alarms.
80 amp 30 ma RCD
32 amp MCB for cooker
32 amp MCB for downstairs sockets
6 amp MCB upstairs lighting
6 amp MCB spare
80 amp 30 ma RCD
20 amp MCB for Boiler
20 amp MCB for Washing Machine & Dishwasher
32 amp MCB for upstairs sockets
6 amp for downstairs lighting

I have 2 questions:
1. On the downstairs Ring and Radials I am getting:
Continuity okay but Radial R1 + R2 are 0.00 ohms and
Ring Main R1/RN/R2 are 0.00 ohms and R1+R2 reads 0.03 ohms.
I have re-tested 2-3 times but still get the same readings.

2. I have done Zs on all circuits and all readings are around 23.0 ohms.
All these readings are a lot higher than the max readings in table B6 of the On
Site Guide BS 7671:2008 (2011)

Any help would be much appreciated.
For your information my Tester is a MEGGER MFT 1710.

Regards,
Ken Ruck.

 

[Jdsparky]

Are you accidentally pressing the button to test continuity....if you are you are nulling the leads to 0

 

[Jdsparky]

And zs readings are ok if it is on a tt installation, if not you need to make a phonecall to dno

 

[telectrix]

what's your Ze and what earthing system? 23 ohms is not acceptable for a TN system.

 

[KENRUCK1956]

Yes it is a TT earthing system with 16mm Main Earth/10mm main bonding and 4mm supplementary.
I understand what you mean by pressing the test button, will check again and let you know.

 

[KENRUCK1956]

It is a TT system and the Ze is 105 ohms.

 

[telectrix]

so. it appears that you are getting the better part of your earth impedance from the service pipes ( water/gas). i'd be inclined to attempt to improve the Ze (Ra) by banging in another rod, but make it 5/8" screwed rods for depth.

 

[Plonker 3]

OP I understand you want to better yourself, but why the hell have you picked a earthing system you seem to have no knowledge of to carry out a install on?

 

[telectrix]

a bit harsh, dilb. he's measured Ze, seems to know it's quite common to have that sort of reading on TT. he's just goofed a bit with his continuity readings. sometimes, with short circuits, it's better not to null the leads first, but measure the circuit, then subtract the lead resistance.


edit: by short circuits, i mean circuits that are of a short length, not with a fault on them. LOL.

 

[Plonker 3]

Harsh maybe, but he doesn't understand what the max Zs values should be for it he is using the wrong table. Things like that we're pointed out to me when I sat my 17th edition exam, yes I shouldn't known everything published in it but looking at the wrong table which clearly states for a TN system not a TT.

We go on about the industry being diluted by lack of experience and knowledge, this is a good example of that, if they had come on before requesting info about it with their own ideas then at least it would show they wanted to learn. But no instead oh it's gone wrong the Internet will help me mentality has kicked in.

 

[KENRUCK1956]

Thanks for all your advice, positive and negative.
We have all got to start somewhere, this is a new career for me and wanted the help of experts to get a good start.
For your information I chose this system because the previous one was unsafe and needed replacing badly.

 

[telectrix]

when you are using RCD s for fault protection, as in this case , the max. Zs values are obtained from table 41.5, BGB. but, i'd still try and reduce the Ra. when eng54 breads this thread he'll tell you that you should be able to attain a sub-1 ohm value with some effort. LOL.

 

[MDJ]

I chose this system because the previous one was unsafe and needed replacing badly.

really? how?

 

[telectrix]

Thanks for all your advice, positive and negative.
We have all got to start somewhere, this is a new career for me and wanted the help of experts ly.

to get a good start.
For your information I chose this system because the previous one was unsafe and needed replacing badly.[/QUOTE]

surely replace it goodly. looking at your layout of circuits, i can't find fault there. good division of circuits. be an idea if you could post the r1,rN & r2 readings for the 2 rings as well.

 

[Deleted member 9648]

Would be a good idea to replace the 100a main switch on your CU for a 100a 100ma S type RCD main switch. Currently you are dependant on the 30ma RCD's for earth fault protection,a 100ma main switch gives back up if they fail to operate. That said the CU layout and Ze value you have complies with the regs for a TT earthing system.