Supplementary bonding for dummies

[majoreyeswater]

Hello all.

I'll start by saying i'm a diy'er, but have a lot of interest in electrics and have been reading up on some things.

One thing i am struggling with is supp bonding.
I understand it is there to limit touch voltages by keeping exposed parts at the same voltage but cannot get my head around this scenario - please stick with me!

Ok, we say bond the lights cpc to the water pipe to the ring cpc in bathroom, if an earth fault developed on one of them, there would be little potential difference if someone was touching two of the bonded areas - so no shock..

BUT, what if someone was out of this 'zone ' in another room, but touching the same water pipe, metal light or metal appliance - would they receive a shock?

An explanation would be great.

Many thanks.

 

[telectrix]

by connecting all the exposed metal parts and extraneous parts ( water/gas pipes ) together, they are all held at the same potential, normally 0V wrt earth. if a fault occurs, and an exposed metallic part becomes "live" by rising to 240V, then all the exposed metallic parts and extraneous parts rise to the same 240V for the duration of the fault ( when the fuse/MCB trips out). that means that if you were in contact with the fault and the mettallic pipes etc. simultaneously, there would not be a potential ( voltage) across your body.

 

[Richard Burns]

Supplementary bonding is installed to protect a specific location, normally a location where there is an increased risk of electric shock, such as in a bathroom where someone can be covered in water decreasing the resistance of an electrical connection the body.

In areas where the risk of electric shock is lower then the only required bonding is main bonding.

Main bonding will reduce touch voltages to a low level but not to so low a level as supplementary bonding.

For example if the touch voltage is 40V in an installation with main bonding a high touch contact resistance may reduce this voltage to 15V.
In an area protected by supplementary bonding the lower touch contact resistance would mean that without supplementary bonding the touch voltage would be 40V but the supplementary bonding reduces this to 10V and so is similarly safe to the main areas of the installation.

 

[majoreyeswater]

Thanks both.

What i cant understand though is if i was, for instance, touching the downstairs taps that are linked to the shower water feed in the bathroom. If a fault occurred on the shower making the water pipe live, how would i not be shocked downstairs touching the taps that are linked?

Thanks in advance.

 

[Deleted member 9648]

Thanks both.

What i cant understand though is if i was, for instance, touching the downstairs taps that are linked to the shower water feed in the bathroom. If a fault occurred on the shower making the water pipe live, how would i not be shocked downstairs touching the taps that are linked?

Thanks in advance.

Because it's likely you would be stood on an electrically insulated floor (wood/carpet/lino etc) wearing insulating footwear,therefore there is no path through your body for current to flow from the 'live' tap to earth. If you were simultaneously in contact with the 'live' pipe and other services or earthed metal then due to bonding then there will be no significant voltage differences and the shock risk is minimal.