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)

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)