Automatic disconnection cannot be achieved in the time required

One example might be IT equipment (as in data processing equipment) fed from an uninterruptable power supply (UPS) - UPSs can operate in several different modes - such that the loads might be supplied by direct mains at some times, from battery at some others and perhaps mains but via a transformer at others again. As a result it's impossible to guarantee satisfactory Zs at all times (when running from batteries there'll almost certainly never be enough current available even during faults to open overcurrent devices promptly), Even RCDs aren't necessarily helpful as you would need a N-PE link on the output side of the UPS for them to provide ADS when the UPS is running on battery, but in other modes such a link isn't permissible.


As a result we look for other methods of providing shock protection - sometimes that'll be adding supplementary bonding, other times relying on the supply voltage to collapse to a safe level if there is a L-PE fault, other times ensuring that the system runs as a separated or unearthed (IT earthing) system when ADS isn't operational. Which (or which combination) depends in the fine detail of the circumstances (e,g. the UPS's design).


   - Andy.

One example might be IT equipment (as in data processing equipment) fed from an uninterruptable power supply (UPS) - UPSs can operate in several different modes - such that the loads might be supplied by direct mains at some times, from battery at some others and perhaps mains but via a transformer at others again. As a result it's impossible to guarantee satisfactory Zs at all times (when running from batteries there'll almost certainly never be enough current available even during faults to open overcurrent devices promptly), Even RCDs aren't necessarily helpful as you would need a N-PE link on the output side of the UPS for them to provide ADS when the UPS is running on battery, but in other modes such a link isn't permissible.


As a result we look for other methods of providing shock protection - sometimes that'll be adding supplementary bonding, other times relying on the supply voltage to collapse to a safe level if there is a L-PE fault, other times ensuring that the system runs as a separated or unearthed (IT earthing) system when ADS isn't operational. Which (or which combination) depends in the fine detail of the circumstances (e,g. the UPS's design).


   - Andy.