Is There A Requirement To Main Bond Back To Met In House?

[flyingspark]

I have just replaced an old fuse board with a dual rcd unit in my house, all ways have additional RCD protection.

The garage is supplied via 20A a radial circuit (installed 2.5mm t&e), cpc continuity, Zs and rcd opening times, etc are all fine.

The garage roof trusses are steel, and linked together by a steel frame, the steel does not extend down to the ground, the walls are solid pre fabricated 1950's type material.

House supply is TNS.

Is there a need to run a bond back to the MET in the house?

Thanks for reading.

 

[Sidewinder]

Are the metal parts extraneous or exposed?

What is the resistance to earth of these parts?

 

[steptoe]

you could make it a very simple solution.........

 

[Andy™]

if its extraneous, then yes. if it isnt, then no

 

[Sidewinder]

I wasn't asking you Andy!  :slap

 

[kerching]

I wasn't asking you Andy!  :slap

Maybe, just maybe, he chipped in thinking this was a question regarding a CPC?............aaaaaaaah. CPC, I was in there only this morning

Just......rubbinitin!

(Sorry)

 

[flyingspark]

Many thanks for your replies.

I do not think the steel roof frame can be extraneous, it sits on top of concrete walls above head height and it does not go downwards to the ground/earth. Concrete as far as I am aware does not conduct 230v right?

There are no metallic gas or water pipes entering the garage, or metallic parts connecting the ground to the steel frame.

If anything, perhaps it could be considered to be an exposed conductive part, although not 'equipment', if the sheathing and insulation were to fail through damage or decay etc, it potentially could become live, assuming the CPD and/or RCD both failed to operate.

So....still undecided/unclear to me if a main bond back to the met would be necessary.

Any other views or definitive answers welcome please.

Thanks again.

 

[Manator]

Concrete does indeed conduct electricity and there is a formula for calculating this ;

[SIZE=11pt]  P=2πa V/I       [/SIZE]

 

[steptoe]

ok,

my simple solution,

would be to treat it as exposed, and bond it back to the garage CU,.

 

[Sidewinder]

Test it to see?

 

[flyingspark]

Update, I have just received my 50 meter wander lead in the post (my 30 meter was too short),

Continuity between MET and the steel roof frame is 67k ohms (67,000 ohms),

IR at 500vdc was 02 M ohms, (20,000ohms) initially my Megger MFT1553 would give a reading of 0.00, but after three attempts I got the .02 M ohms.

Anyone here use a Megger MFT1553 that has experienced this with IR?

I have seen various figures posted in different places, one of which is 6,667 ohms, ((Rcp>(Uo/Ib) - 1000)) now, if I use this figure (have 30mA RCD's on board, I don't need to bond back to MET, but if I take the 23,000 ohms then I do.

I called NICEIC tech help, and was told that the continuity test of 67,000 is way over, so no need to bond, but I forgot to ask which figure he was referring to.

I am confusing myself now..............

Many thanks

 

[Andy™]

iirc, its 22k ohms

 

[flyingspark]

Many thanks to the helpful replies

To conclude,  Bonding back to MET is not required as continuity between garage roof steel structure is 67k ohms, so over the 22k or 23k ohms, and so proved to not be extraneous.