Bonding

You can introduce a (small) risk of shock that could otherwise be avoided.

In general if someone for whatever reason ends up in contact with a hazardous live part (in theory shouldn't happen, but in the real world it can - e.g. from a damaged flex or faulty Class I appliance where ADS has failed for some reason (e.g. broken c.p.c. or failed RCD on a TT system)) then having an earthed part to hand means the circuit can be completed and they receive a shock - whereas if the part had been left floating, they'd feel nothing (like a bird perched on a bare overhead line).

For parts outside of the equipotential zone there's the additional risk of having two different "Earth" potentials to hand (especially nasty when PME earthing is used (during an open-PEN event), or a TT system fails to disconnect on an Earth fault when it should).

The risks are generally small, certainly smaller than not bonding when it is required, but if it's easy and cheap to avoid (and not bonding something is usually pretty cheap and easy) most people would welcome the additional safety, however small the benefit.

   - Andy.

You can introduce a (small) risk of shock that could otherwise be avoided.

In general if someone for whatever reason ends up in contact with a hazardous live part (in theory shouldn't happen, but in the real world it can - e.g. from a damaged flex or faulty Class I appliance where ADS has failed for some reason (e.g. broken c.p.c. or failed RCD on a TT system)) then having an earthed part to hand means the circuit can be completed and they receive a shock - whereas if the part had been left floating, they'd feel nothing (like a bird perched on a bare overhead line).

For parts outside of the equipotential zone there's the additional risk of having two different "Earth" potentials to hand (especially nasty when PME earthing is used (during an open-PEN event), or a TT system fails to disconnect on an Earth fault when it should).

The risks are generally small, certainly smaller than not bonding when it is required, but if it's easy and cheap to avoid (and not bonding something is usually pretty cheap and easy) most people would welcome the additional safety, however small the benefit.

   - Andy.