Lack of supplementary bonding - what’s the danger ?

[JohnW2]

Yes penny has dropped . The supp bond closes the loop in the earthing arrangement. I see it now. Thanks for being patient.

No problem - we've all been there

How is an RCD going to help in that scenario? (Without supp bonding) It isn’t is it ?

Yes, of course it will help (will trip if 30 mA or more goes through the victim's body).

Some of the current arriving in the L conductor will return to earth via the victim's body rather than returning in the neutral conductor. The current in the N going through the RCD will therefore be less than that in the L going through the RCD - and it is that ';difference'; (the 'residual current') that will cause the RCD to trip

Which begs the question why is it (sup. Bonding) permitted to be omitted if it’s RCD protected and the other criteria are met ?

... because it does work - as above.

 

[equitum]

“As I've said, the only thing that will remove that risk is supplementary bonding within the location.”

Johnw2 your comments above - so anRCD will not ? Why ?

 

[JohnW2]

“As I've said, the only thing that will remove that risk is supplementary bonding within the location.”

Johnw2 your comments above - so an RCD will not ? Why ?

An RCD will work to limit the duration of a fault (hence the potential duration of an electric shock).

What an RCD cannot do is to prevent touch voltages arising (before the RCD operates) which are high enough to prevent potengtially serious electric shocks occurring - as I said, only supplementary bonding can do that.

...so, if you don't mind getting an electric shock and are prepared to trust an RCD to limit the duration of the shock to an adequate extent to prevent you dying, then an RCD is adequate. However, if you'd prefer not to have an electric shock in the first place, you would need supplementary bonding, even if there was RCD protection.

 

[equitum]

Won’t an mcb work to limit the same duration - I.e operate within 0.4 seconds in the same scenario?

 

[JohnW2]

Won’t an mcb work to limit the same duration - I.e operate within 0.4 seconds in the same scenario?

That depends. If the fault is a 'dead short', or something approaching that, then (in a TN installation) it will cause an MCB to operate immediately, nearly always before anyone has a chance to get a shock.

However, if there is a fault of significant impedance, an MCB will take a lot longer to operate (if ever). Furthermore, if the only 'fault current' is that through a human being (who has simultaneously touched L and something earthed), then that current would be so tiny (even if 'lethal') that an MCB would never operate, so the shock would persist indefinitely.

 

[trojanhawrs]

Furthermore, if the only 'fault current' is that through a human being (who has simultaneously touched L and something earthed), then that current would be so tiny (even if 'lethal') that an MCB would never operate, so the shock would persist indefinitely.

Supplementary bonding would not protect against this scenario though.

 

[flameport]

Won’t an mcb work to limit the same duration - I.e operate within 0.4 seconds in the same scenario?

MCBs used for circuits with 0.4s disconnection time.
An RCCB will disconnect in 0.04s, 10x faster.

Some circuits can have up to 5s disconnection time.

 

[equitum]

MCBs used for circuits with 0.4s disconnection time.
An RCCB will disconnect in 0.04s, 10x faster.

Some circuits can have up to 5s disconnection time.

No circuits that supply a bathroom are permitting have 5s disconnection time

 

[flameport]

A fault elsewhere in the installation will cause a voltage on anything connected to the MET including what's in the bathroom.

 

[JohnW2]

Supplementary bonding would not protect against this scenario though.

Perhaps I wasn't clear, but my "furthermore" was meant to indicate that II wasn't suggesting that it would - merely that this was another alleged advantage of having RCD protection

 

[JohnW2]

A fault elsewhere in the installation will cause a voltage on anything connected to the MET including what's in the bathroom.

Yes, but that will (equally) affect everything in the bathroom which is connected to the MET and, unless there is a fault on one of the circuits serving the bathroom (such that fault current is flowing through its, and only its, CPC) there will be no PDs between anything and anything else within the bathroom - even if it take 500 seconds (or 5 hours ) for the fault elsewhere in the installation to be cleared..

... and I've already discussed the situation in which there is a fault on a circuit serving the bathroom, in which case it's CPC and exposed-cps will rise to a potential much higher than anything else in the room (not part of the same circuit) which is connected to the MET

 

[equitum]

Couldn't find any suitable diagram so - is this any help?

What will be the voltage between points A and B with and without the supplementary bonding.

View attachment 342072

Still thinking about this !!! You have conveniently drawn the supp. bond at the point of touch, let’s say the tap. If the supp. bond was connected at say the floor level, electrically this is no different to without supp. bonding in place in your picture other than a negligible additional resistance of the pipe work /cpc/meb is it ?

 

[trojanhawrs]

Think of yourself as a resistor. In the first picture you're the highest point of resistance between the voltage on the enclosure and the taps path back to earth. Meaning you can consider the volt drop to be entirely across your person.
In the second picture you're a parallel path, meaning you have to consider the entire cable as a resistor. The voltage will drop equally across the whole path back to earth meaning there will be little difference between the voltage at the fault and at the tap.

 

[equitum]

Think of yourself as a resistor. In the first picture you're the highest point of resistance between the voltage on the enclosure and the taps path back to earth. Meaning you can consider the volt drop to be entirely across your person.
In the second picture you're a parallel path, meaning you have to consider the entire cable as a resistor. The voltage will drop equally across the whole path back to earth meaning there will be little difference between the voltage at the fault and at the tap.

That’s assuming the bond is made at the tap itself other than in reality some other point in the pipe work

 

[trojanhawrs]

That’s assuming the bond is made at the tap itself other than in reality some other point in the pipe work

Closer the better, but any link is going to reduce the potential between the 2 points.