UPS floating

Question

(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 &lsquo;mains&rsquo; via a single pole C16Amp 30mA RCBO 
 When the UPS is in &lsquo;bypass&rsquo; or &lsquo;online&rsquo; 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&rsquo;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&rsquo;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&rsquo;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!

gkenyon · Accepted Answer

Udtwmc said: I’m also interested to know if the UPS (when in battery mode /floating L-N) is a true IT system and can be treated as such? 
 Only the manufacturer can answer that. 
 
 Udtwmc said: I’m certainly looking at fitting an RCD to cover a secondary fault although I would want to avoid nuisance tripping and there are a couple notes under 411.6.3 that are putting me off using one. 
 You'd certainly need to think about protective conductor currents and as such the RCD may operate on first fault. 
 
 There are a couple of other problems: 
 1. If the plug is removed, equipment will still be powered but there will be no connection of the PE conductor to Earth as well. 
 2. If the supply to the premises is TN from a distributor, consider Regulation 551.4.3.2.1. You can't rely on the distributor's means of earthing for such a UPS, because if the distributor is working on their cables, the connection with Earth may be removed. To fix 1. , you would need a separate earthing connection from the distribution board (or MET if you want to address the risk of electrical maintenance removing the PE connection to the distribution board without powering down the UPS - there are some important issues here for the CDM risk assessment). 
 To fix 2., you can provide a consumer's earth electrode connected to the MET. An additional consumer earth electrode is actually recommended for TN systems in any case, see Regulation 411.4.2.

gkenyon · Answer

Udtwmc said: what purpose does the earth connection of the exposed conductive parts to the upstream distribution earth or building MET actually serve? 
 Very simple. It provides a means of connecting "first fault" to Earth, so that either "second fault" is detected, or an IMD can operate on "first fault". 
 Further, regards the statements I made re Electrical Separation vs IT, if there is no Earth connection to PE, it can't be an IT system, which means it's only safe (under all conditions) to supply a single item of equipment, unless the installation is under constant supervision of skilled or instructed persons. 
 This is the important differentiator between IT and Electrical Separation.

gkenyon · Answer

AMK said: I’m going with a TN system with electrical separation as the output of the UPS in battery mode is floating with respect to earth, but the exposed conductive parts of the equipment are still earthed. 
 If the exposed-conductive-parts are still Earthed, that would be an IT system, not electrical separation. [And in this case, you can't rely on the distributor's means of earthing as discussed above.] 
 In electrical separation, there is no connection between PE [exposed-conductive-parts] and the general mass of Earth.

gkenyon · Answer

Udtwmc said: And regarding the point on regulation 551.4.3.2.1 I read this as only being relevant when the generating set (my UPS) provides a supply for 'distribution to the public' ... 
 Nope, it's when the input to the UPS is connected to a public supply, and effectively the bypass is used to supply the downstream ("UPS supplied") equipment from the public supply. In that case, the bypass switching is the equipment that switches between UPS output and public supply - the UPS output is the "switched alternative".

UPS floating

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