Cross Bonding

[Gravid]

Hey up, I understand that if disconection times can be met there's no need for supplementry bonding,

but is it still necessary to cross bond say pipes in an immersion heater cupboard or underneath a boiler?

I don't mean coming off a lighting circuit with 4mm, but just linking them together with 4mm. Was

this ever a requirement in the regs ? Cheers.

 

[scotsparky]

Have a read of 411.6, 415.2 in the regs which may help

 

[Gravid]

So reg 411.3.1.2 about covers it then, protective equipotential bonding including other installation pipework and

ducting. So you are supposed to bond all the pipes in that situation regardless of disconnection times.

 

[scotsparky]

411.3.2 is your Main equipotential bonding where the servives enter the building that HAS to be bonded.
At other pipework is Suplimentary Bonding which is different and is covered in section 415.2

Have a look at Fig 2.1 0n page 32 of the regs which may explain the 4 different names for protective conductors and what they mean.

 

[Gravid]

Oh aye yeah, my mistake, thats really meant for Oil or another incomming service.

In fig 2.1 its saying where required to supplementry bond exposed conductive parts, i thought boiler/immersion pipes

where extraneous. In 4.6 of the OSG it says where requirements have been met ( does this mean ADS? ) theres no

need to bond. Probably confusing myself thanks any way.

 

[scotsparky]

i have quotes from the 17th regs book not the OSg

Yes ADS as well as R,50 /Ia where Ia is the rated current of the overcurrent device or the tripping current of the RCD,RCBO
As the pipework is normally linked to a boiler then it is tied to the electrical system so therefore exposed conductive parts. the only difference my be something like a standpipe in a caravan park.

 

[Gravid]

Ok, but i still can't find anything that says you have to do this, I mean if your main bonds are in place at your incoming

water and gas/oil why do you bother linking random pipes with 4mm? Just i've worked with people who do and people

who don't and was looking into whether this was a reg.

 

[scotsparky]

It is not a clear Yes or No it all depens on the installation.

Using the Long lead method from the main earth terminal test to exposed conductive parts in the installatio. In this case the Pipework below the boiler. If the resistance is less then 50/ Ia then no bonding is requierd as you have the disconnection time as well as no more than 50V on the exposed metal. If it is higher then you have to install suplimentary bonding.

The Electrical safety council say as a rule less then 0.5 ohms. As most houses have a hundred amp main incommer so 50/100 =0.5 so your covered.

hope this makes sence its is all explained in the Reg 415.2. also read reg 544

 

[Doug]

This is what the ESC & others have to say about supplementary bonding at boilers:

Does boiler pipework need to have additional equipotential bonding for electrical safety reasons?

There is no specific requirement in the Regulations for boiler pipework to be supplementary bonded. However, such bonding may be called for in the boiler manufacturer’s instructions, in which case BS 7671 requires those instructions to be followed (Regulation 510.2 refers). Any stated requirement for additional bonding that is considered to be unnecessary should be queried with the manufacturer concerned, and amended installation instructions requested.

(See reg no 411.3.3)

New or Rewired Domestic and Similar Installations, Question 6

 

[slocm3105]

The Electrical safety council say as a rule less then 0.5 ohms. As most houses have a hundred amp main incommer so 50/100 =0.5 so your covered.

I thought it was 0.05 ohms mate... someone poelase correct me if wrong but im pretty sure its gotta be 0.05 ohms or less.

Cheers

 

[Guest123]

I thought it was 0.05 ohms mate... someone poelase correct me if wrong but im pretty sure its gotta be 0.05 ohms or less.

Cheers

No need to correct you matey, 0.05 it is.

Check GN3 for confirmation.

Scottie is also right as it is to keep the touch voltage thay may flow under fault coditions below 50V.

 

[scotsparky]

sorry typo!

 

[slocm3105]

Nice one. Cheers chaps

 

[[email protected]]

I thought it was 0.05 ohms mate... someone poelase correct me if wrong but im pretty sure its gotta be 0.05 ohms or less.

Cheers

Yes it is ideally less than 0.05Ω (page34 GN3)

Which, if the main protective bonding conductor was 10mm would give a run of up to 27m and still be within the 0.05Ω value.

 

[tomasi]

I always thought the max resistance was 0.05 ohms not 0.5 ohms and Ia in the formula related to
the nominal rated tripping current of an rcd/rcbo and the current that needed to flow to cause operation
of a device such as a fuse/mcb in a given time such as 0.4 seconds and not the nominal rated current of the device.

 

[Scott F]

R<50v/Ia

How do you arrive at the 0.05 figure as Ia being the operating current in amps (like scots spark said would generally be 100 Amps so 50/100 =0.5
I have the regs and GN3 so I know the figure is 0.05
So how does one arrive at the figure?
Cheers Scott

 

[Gravid]

Is it that the 0.05 is what you are hoping to get below if you were to measure the resistance between two pipes, say hot and cold water pipes for example. If its above this then you would have to bond them together. think thats what GN3 is saying on p 35.

This equation, are you supposed to use the cut out or your RCD rating?
If you have a 60A cut out does this mean its got to be below 0.8? (50/60)
and if you do the equation with a 30ma RCD it comes out at 1667 !

To be honest really struggling to get my head around all this !!

 

[scotsparky]

Gravid the 1667 figure look familiar???? it should its the max ZS for 30ma rcds up to 230v (see page 50brb.)
the resistance measured is between the pipe and the MET at the D.B subtracting the long lead and test leads.

Use the RCD tripping currrent when using the formula. As you have the resistance and the In of the protective devics if you transpose your formula to rxIn <50 then you are ok as any voltage present in the metalwork during fault conditions is less than 50v (touch voltage) remember 120v dc


0.05 is normally used it goes on the side of safety as with the tripping times. If you are above 0.05 then either do the maths and prove if it need bonding ok put in supplimental bonding so its under 0.05. ()for all the time it takes i would bond it)

 

[Gravid]

Yeah it did look familiar, TT systems sprang to mind.

 

[scotsparky]

All based on the same theory. do you understand it now?

 

[Gravid]

Yes mate i've got it now, just had to go back to the basics, i understand your making sure the touch voltage in the event of a fault doesn't go above 50v.

 

[scotsparky]

thats the chappy!