Mains bonding query

[Monitor]

Hi all I'm going through my training and am confusing myself in regard to mains bonding. Is it there to protect the instalation from a fault withing the building or a fault outside the building making the incoming water or gas supply pipes live. Sorry to sound stupid but I need to get this clear in my head.

 

[IQ Electrical]

Hi all I'm going through my training and am confusing myself in regard to mains bonding. Is it there to protect the instalation from a fault withing the building or a fault outside the building making the incoming water or gas supply pipes live. Sorry to sound stupid but I need to get this clear in my head.

Main protective bonding conductors connect all extraneous-conductive parts of the installation to the main earth terminal, eliminating potential differences between the parts.

 

[Guest123]

Main Protective Bonding (MPB) is used to connect any extraneous conductive parts which are conductive parts that may introduce a potential, which is generally earth potential, into an installation but do not form part of it i.e main service pipes, structural steelwork etc, to the Main Earthing Terminal (MET) of the installation.

This then creates an Earthed Equipotential Zone by maintaing both exposed & extraneous conductive parts at roughly the same potential so if a fault occurs all metalwork within that building would rise to the same fault voltage so should decrease the chances of receiving an electric shock.

So in answer to your question, bonding of extraneous conductive parts to a buildings MET protects against the appearance of fault voltages from outside of a building.

 

[yellowvanman]

Bonding is there to provide a equipotential zone.

If an appliance become defective (kettle for instance) then it could become live, in which case the earth wire becomes live and as its conencted back to the consumer unit the earth bar becomes live and as its connected to the water and gas etc. then all the water/gas pipes become live. If you pick up the kettle (its live) and you touch the taps (thats live as well) because there is no potential between the live of the kettle and the live of the water pipes you don't get a shock, whilst your waiting for the protective device to trip!

Thats the theory anyway, in reality there is a potentail between the live kettle and taps but its not as much as 230V!

 

[Deleted member 9648]

OK,here goes.
In the event of a fault to earth,all conductive parts of the installation...(read the definition in the BRB)...may experience a rise in potential relative to earth until the protective device clears the fault. A person in contact with a conductive part in such circumstances may be at risk of a shock.
In practice that risk is minimal because within the confines of a building the person is insulated from the general mass of earth by virtue of being on wooden floors,carpet etc.
A hazard arises though by extraneous conductive parts..(read the definition in the BRB)...introducing an earth potential into the building....now a person in contact with both a conductive part and an extraneous conductive part for the duration of the fault would be at serious risk of a shock.
By bonding ECP's to CP's there can be no potential difference between them and the shock risk is removed.
This explains why bonding is carried out at the point of entry of the ECP...as the earth potential comes from outside the building.

 

[ezzzekiel]

and there you have it in a nutshell 4 excellent explanations of why main bonding is carried out,

remember this,

earthing limits the duration of a fault, whereas bonding limits the magnitude.

 

[Monitor]

Thats a great explanation thanks a lot

 

[IQ Electrical]

remember this,

earthing limits the duration of a fault, whereas bonding limits the magnitude.

That's the best way of remembering and visualising the differences!

 

[telectrix]

OK,here goes.
In the event of a fault to earth,all conductive parts of the installation...(read the definition in the BRB)...may experience a rise in potential relative to earth until the protective device clears the fault. A person in contact with a conductive part in such circumstances may be at risk of a shock.
In practice that risk is minimal because within the confines of a building the person is insulated from the general mass of earth by virtue of being on wooden floors,carpet etc.
A hazard arises though by extraneous conductive parts..(read the definition in the BRB)...introducing an earth potential into the building....now a person in contact with both a conductive part and an extraneous conductive part for the duration of the fault would be at serious risk of a shock.
By bonding ECP's to CP's there can be no potential difference between them and the shock risk is removed.
This explains why bonding is carried out at the point of entry of the ECP...as the earth potential comes from outside the building.

that has got to be the clearest explanation i have ever read. should clear up a lot of confusion.

 

[jamminuk]

Hi all, i'm new to this forum, from what i've seen so far it looks like a great place to pick up some good tips and advice. I'm due to sit the 2391 exam next week first attempt, and there's a few thing's i'm unsure on that are bugging me.

Earth-free local equipotential bonding - I understand that the bonding reduces/eliminates dangerous touch voltages but why is it earth free??

Testing of SELV and PELV - Not sure how this is done, am i right in saying that a selv circuit i.e. a lighting circuit is actually on a normal circuit up until the transformer, after which it is then separated, how would you test this? Would it be from the DB using 250Vdc? or would the transformer be linked out? But this would take forever with the downlighters that have a transformer for each one. I've heard of the method of linking the busbar to the neutral bar, this method then allows you to perform an IR test without having to take out any volt sensitive equipment, but will that answer be excepted in the C&G exam??


RCDs - Only deal with earth leakage? if that's the case what's the point of having earth fault loop? is it just extra protection? As if there is a fault to earth the RCD would trip before the fault path would even get a chance to trip the breaker?

There's more there's loads more but these will do for now.

Appreciate any help anyone can give.

Thanks

Ben

 

[Deleted member 26818]

It's not connected to the masive earth.
SELV conductors are tested at 250V d.c.
LV conductors are tested at 500V d.c.
Where there is voltage sensitive equipment, that may be damaged, and it is not practical to disconnect, the test may be conducted between L&N connected together and earth at 500V d.c.
RCDs are additional protection. At present we are still required to design and install circuits so that the earth fault loop impedance is low enough to allow an MCB or fuse to operate.

 

[jamminuk]

Hi Spinlondon,

Thanks for your quick response, with regards to SELV do you mean the secondary side of the transformer at 250v and the primary side at 500v? the way i see it, correct me if i'm wrong but the selv circuit starts after the transformer? the selv doesn't start back at the DB? Also then to IR test between selv circuits and other circuits using 500v? how would this be achived?

The other thing that was bugging, which i only realised after reading through the chief examiners report that there are 2 main types of inspection, the initial verification and periodic, i would not have put that as my answer. What then is the difference between initial virification and initial inspection???

The GN8 is a good reference bookhowever i feel it doesn't necessarily say why things are the way they are, it's just this is the way it is and that t, i need to know why things are the way they are, what would happen if it wasn't like that, i find know that helps me understand why things are done the way they are.

Thanks

Ben

 

[Deleted member 26818]

Yes the primary side would be LV and should be tested at 500V d.c. and the secondary side SELV and tested at 250V d.c.
Where the SELV conductors cannot be removed from the transformer, as is the case with many used for downlights, the conductors are tested to earth at 250V d.c.
The difference between IR testing for initial verification and a periodic, is that with an initial verification, you would be expected to test during installation, before connecting any voltage sensitive equipment.
Obviously with a periodic, that would not be possible.
So you would have to either disconnect at least one of the conductors, or link out the equipment, or test with L&N connected together to earth, or in some cases use 250V d.c.
To prove separation between SELV and LV circuits, you would test between the live conductors of the SELV circuit connected together and the live conductors of the LV circuit connected together at 500V d.c.
There has been some suggestions that 250V should be used when testing between L&N connected together and earth.
To be honest, a fairly pointless exercise, as the point of connecting L&N together is to prevent the test voltage from flowing through the equipment.