Extraneous....Yes or No....

[Guest123]

Ripped form an NIC newsletter.....

The NICEIC are now quoting a figure of 7000ohm (based on 30mA fault current for a disconnection time of 0.4s). It was dicussed in a recent connections magazine and also discussed at one of their tech talks.

From pg 47 in the magazine issue 172 ​

The metallic part can be assumed not to be an extraneous-conductive-part if the following condition is met:
Rcp> (Uo/Ib) - Ztl
where,
Rcp is the resistance between the conductive part and the MET in ohms
Uo is the nominal voltage to Earth in volts
Ib is the value of current through the body in amperes that should not be exceeded. (The value may be taken as 30 mA for a disconnection time of up to 0.4 s, as given in DD IEC/TS 60479)
Ztl is the impedance of the human body in ohms. The value suggested in DD IEC/TS 60479 is 1000 ohms where Uo is 230 V (50 Hz) under dry or wet conditions.
Taking Ib as 30 mA and Ztl as 1000 ohms (as suggested above where the disconnection times in the installation are 0.4 s or less and Uo is 230 V), the limiting value of Rcp is given by:

Rcp > (230/0.03) - 1000
Rcp > 6667ohms

Thus, if Rcp exceeds 6, 667 ohms, the pipe may be considered not to be an extraneous-conductive-part, such that main bonding of the pipe is not required ​

 

[Spudnik]

Nice one bud.

 

[ian.settle1]

NICEIC may say that but if someone was hurt because you followed that you could be batting on a sticky wicket as if it is not in the regs?

 

[Spudnik]

Interesting point Ian, but i think the NIC would have responsibility if anything did happen, especially as they have published it.

 

[WayneL]

Hi,

GN8 gives guidance on this using what they term the 'let-go' threshold, quoted as 10 mA.

Using the same equation this gives a value of 22 kohms for the threshold - above this, you would not consider the metalwork extraneous.

It is really a design issue and up to the designer which value of current he doesn't want to exceed (e.g. 10 mA).

Where the NIC get off declaring something like this as fact, I don't know.

I'm sure they help create half of the misunderstanding of the regs that exists.

GN8 goes on to say...."the designer would need to consider possible variations in resistance and whether a lower limit on the current flowing through the human body or livestock is necessary"

With this in mind, I think I would be tempted to err on the side of caution and use a lower figure for my current - i.e. 10 - 15 mA, and go for a higher threshold.
Anything lower than the threshold obviously should be bonded.

 

[hd5178]

Interesting point there...Might have to have a read up myself on that!

 

[IQ Electrical]

I mentioned that I use 23K Ohm, that's my value

 

[ezzzekiel]

thought an ir test between earth and unknown part had to be less than 20k ohms to be considered extraneous, although i cant find a reference to it now

 

[telectrix]

so the nice ppl are thinking 6. 6k, and intelligent life thinks 20-23k. i know which figure i will be using!

 

[sjm]

thought an ir test between earth and unknown part had to be less than 20k ohms to be considered extraneous, although i cant find a reference to it now

I seem to remember being told by NIC that a value over 0.25 megaohms would mean that the unknown part would not be an extraneous conductive part.

 

[IQ Electrical]

Well I've seen the 22 and 23 but never a 25K value!
I'll stick with my 23K

 

[Guest123]

I'm with GN8 too.

 

[Des 56]

They will have to bring out a guidance book fairly soon with the way they are going and the numerous NIc regs

Its just been on the news
The Niceic has staged a coup d"etat at the IEE building in London
The present IEE administration has been given a visual condition report and found to be out of touch with insanity and electrocuted with an extraeneous plastic pipe
Tony Cable,the spokespark for the Niceic said all rebellious other tribes will now be crushed in the next few weeks

Always knew they had a need to be the dictator

 

[sjm]

OK guys perhaps I wasn't totally clear in my post.
The post I was replying to was talking insulation resistance (ir) test between the MET and unknown part hence the value of 0.25 megaohms.
Hope that's now as clear as mud.
Like the news flash Des.

 

[Dave_]

hello everyone, im new to the forum and this is my first post.... just been reading this thread and have a little thing to add....

Surely if the metal pipe has a high resistance to earth it would not be a bad idea to bond it as it would be sitting at around 230v under fault conditions?

or am i just stupid? haha.....