I suppose that where the utility services are still metalic then main protective bonding would be required and where there is a chance of shock from a circuit fault or from a broken PME supply neutral (if it exists) then the supplememtary bonding is to guarattee that the fault voltage within the special location doesn't rise higher than a voltage that would be likely to kill or cause a serious injury.

Legh

During a fault and thus while there is a very large short circuit current flowing down the cpc, a large voltage will develop along the cpc - thus the casing of equipment A may be at half the supply voltage relative to the local potential in the extr-c-p D. So if someone touches the pipe and the casing simultaneously, they may die. By locally bonding the casing to the pipe, they will end up at much the same potential and thus no death. In this latter case, fault current is flowing down the cpc and the pipe, and thus there is a similar voltage drop along both of them.

As a bit of background, we generally need circuits to disconnect within 0.4s or less* if we're to offer good protection from electric shock - but some circuits can take far longer to disconnect - e.g. 5s* in the case of distribution circuits, or socket circuits rated over 63A or other circuits over 32A.


As others have said, during an earth fault - before the circuit protective device finally disconnects - exposed metalwork on that circuit can be raised to hazardous voltages.


By and large the risks from these longer disconnection times are small, and usually mitigated by main bonding, but in some special locations like bathrooms (no footwear and wet skin) the risks can be far higher so some additional precautions are needed. Especially if you had several circuits feeding the bathroom and one of them had a longer disconnection time (say a 50A circuit for a shower) then you could have a quite lethal voltage between metalwork connected to the circuit and metalwork connected to other circuits (or extraneous-conductive-parts). Supplementary bonding makes sure that all such parts are connected together locally - so the voltage differences within the bathroom are limited (they're never entirely eliminated - but the rules for supplementary bonding make sure that the combination of the voltage difference and duration aren't normally hazardous).


Where you can be sure that everything in the bathroom disconnects quickly and no hazardous voltages can be imported from elsewhere in the installation - the supplementary bonding can these days be omitted from bathrooms.


   - Andy.


(* 0.4s and 5s are for TN systems, the equivalent for TT systems are 0.2s and 1s)

The issue is that we need to bond to everything that could bring a real earth potential into the bathroom. During a fault the local earth voltage may be very different to the real earth potential and so everything that could import the real earth potential needs to be bonded so that everything that a person could touch is at roughly the same potential.