UK Scenario where lack of supplementary bonding is dangerous

[Falkus]

My take on supplementary bonding:

I don't think it's intended as a backup CPC, although it can achieve that. Its full name "supplementary protective equipotential bonding", and that it includes extraneous parts, leads me to conclude that its main goal is equal potential. The formula we need to meet is:

R <= 50V/Ia (Ia= operating current of protective device)

When a fault to earth occurs, a circuit is completed through which a current flows. As long as this current flows, there will be a voltage drop between the site of the fault and the MET, so there will be a potential difference between the site of the fault, and all metalwork connected to the MET. The aim of supplementary bonding, as I understand it, it to keep that potential difference to no more than 50V.


Example: electric shower without RCD protection, on a B40. Extraneous pipework in touching distance, connected to the MET via main bonding.

Fault of negligible impedance occurs at shower, 200A flows. There is a voltage drop between the shower and the MET, and therefore a potential difference between the shower and the nearby pipework.

50/200 = 0.25ohms

As long as the resistance between the shower and the pipework is 0.25 ohms or less, then the voltage between them will be no more than 50V.


The above example is for a fault that caused instantaneous disconnection. Supposing a tenuous fault occurred that didn't cause instantaneous disconnection. Let's use a fault current of 199 ohms, as this is right on the edge:

50/199 = 0.251 ohms

So as the fault current gets lower, the max resistance between the shower and pipework to keep the potential down to 50V gets higher. If you meet the formula in 415.2.2, then any fault will either instantly disconnect, or will be kept to 50V or lower.

The 50V rule makes sense. In any case a fault current (not overload) will take out the OCPD in 0.4s provided max Zs has been met. So either a short or Ipf of 200ohm and the OPCD surrenders instantly or you’ve got up to 0.4 seconds of pain between 50V and 240V on a linear scale with reference to the resistance between the extraneous conductive part to the MET and the fault current ?

 

[timhoward]

@Pretty Mouth thanks for taking the time to write a textbook answer with good examples.
With so many installations having Dual RCD boards or RCBO boards it's scary how easy it is to forget the details of regs that don't need to be regularly considered day in and day out.

 

[Pretty Mouth]

The 50V rule makes sense. In any case a fault current (not overload) will take out the OCPD in 0.4s provided max Zs has been met. So either a short or Ipf of 200ohm and the OPCD surrenders instantly or you’ve got up to 0.4 seconds of pain between 50V and 240V on a linear scale with reference to the resistance between the extraneous conductive part to the MET and the fault current ?

I think the point of sup bonding is when a fault occurs that doesn't produce enough current to operate the device in the required time, so the victim could be subjected to a voltage for longer than 0.4 seconds. I assume that the sorts of faults that it deals with are considered tolerable in general, but not in the high-risk situations that SB is required (eg bathrooms).

I suppose this would be faults where there is not a direct metal-to-metal short. Perhaps a heating element that has corroded and is connecting to earth through moisture may be of higher impedance, may take some time to disconnect. Or perhaps if there was a problem with low supply voltage.

 

[Pretty Mouth]

@Pretty Mouth thanks for taking the time to write a textbook answer with good examples.
With so many installations having Dual RCD boards or RCBO boards it's scary how easy it is to forget the details of regs that don't need to be regularly considered day in and day out.

You're welcome. I found that it isn't very well explained in the textbooks that I have read, plus it is difficult to get the head around anyway, which may be why many of us aren't 100% on it.

 

[Falkus]

I think the point of sup bonding is when a fault occurs that doesn't produce enough current to operate the device in the required time, so the victim could be subjected to a voltage for longer than 0.4 seconds. I assume that the sorts of faults that it deals with are considered tolerable in general, but not in the high-risk situations that SB is required (eg bathrooms).

I suppose this would be faults where there is not a direct metal-to-metal short. Perhaps a heating element that has corroded and is connecting to earth through moisture may be of higher impedance, may take some time to disconnect. Or perhaps if there was a problem with low supply voltage.

I’m still not convinced that with the parallel paths of the ECP and CPCin a dwelling there would be any significant PD take a standard 3bed semi with the CU under the stairs , this is no further than the loft or airing cupboard In practice from the bathroom in many cases

 

[Pretty Mouth]

I’m still not convinced that with the parallel paths of the ECP and CPCin a dwelling there would be any significant PD take a standard 3bed semi with the CU under the stairs , this is no further than the loft or airing cupboard In practice from the bathroom in many cases

I agree, as in post #2

 

[mainline]

@Falkus and others, out of interest how would you code this one that I’m about to do:

The bathroom light needs changing anyway as it isn’t IP rated.

Must be a very low ceiling to code it for not being ip rated

 

[nicebutdim]

Must be a very low ceiling to code it for not being ip rated

Or a light which drops below 2.25m. I see plenty of pendant sets fitted in new build bathrooms.

 

[mainline]

Or a light which drops below 2.25m. I see plenty of pendant sets fitted in new build bathrooms.

There are plenty of ip rated pendant lights

 

[timhoward]

Must be a very low ceiling to code it for not being ip rated

It is, and I'm not exactly short of reasons to condemn this light!