(UK based industrial equipment to comply with BS 7671, operated by skilled authorised personnel.)
I have several class I devices that are supplied via a 230VAC 2200VA UPS which itself is supplied from the ‘mains’ via a single pole C16Amp 30mA RCBO
When the UPS is in ‘bypass’ or ‘online’ mode the upstream mains supply is the primary supply source and protection against electric shock is achieved through the usual means of basic protection and automatic disconnection of the supply.
When the mains circuit breaker is off or has tripped due to an earth fault the UPS battery and inverter take over as the new supply source (unsurprisingly), but now none of its load side live conductors are bonded to earth. The UPS will continue to detect short circuits and overloads and consequently shut itself down but obviously won’t detect a single fault to earth.
Does this installation effectively become an IT system when the UPS is the supply source and should I make use of various monitoring devices (411.6.3) or is the installation forever deemed a TN system and fault protection switches from ADS to electrical separation? I’m struggling to see how I can comply with the latter (see 413.3.6) because the exposed conductive parts of the class I devices are permanently bonded to earth.
The UPS isn’t supplying emergency services, just a request by the customer so as to keep some basic control equipment operating and logging data whilst the plant shuts itself down.
Advice always greatly appreciated!