Isolation Transformer and its Secondary Protection

Question

When using an isolation transformer in a control panel. in this case for a motor heater, I think I should probably tie 0V to earth within the control panel. 
 - I would also like to protect the secondary, however what are the rules here, do you have to provide 2P MCB? 
 -Obviously I have an upstream 2P MCB of the transformer. 
 - I would think its perfectly OK to use only a single pole on MCB on the line side output of the transformer? 
 
 Does anybody have any comments on this?

AJJewsbury · Accepted Answer

Firstly this might be covered by more specific standards than BS 7671 - which might differ in some details. 
 From a general/BS 7671 perspective though, there's probably a bit more to consider... 
 1. What sort of voltages for the 2ndary are we talking about? (anything from 12V to 400V are seen in various applications) 
 2. Are there any functional requirements about earthing? 
 3. Are there any requirements for resilience? (e.g. while some systems will happily disconnect on 1st fault, others (e.g. some safety critical) are intended to keep working safely on 1st fault) 
 4. What's the intended approach for shock protection? SELV, PELV, FELV, ADS, separation, double/reinforced insulation? 
 Once you've decided all that it'll probably drop out whether the secondary should be earthed or not, and then how many poles you need to provide protection for. Generally for overload protection you need only protect n-1 of the live conductors, and for ADS with an earthed neutral generally only the line conductors need disconnection - but there are exceptions in specific circumstances. 
 - Andy.

mapj1 · Answer

What you do on the secondary side depends on the reason for the transformer, the transformer behaviour under overload, and the kind of load connected. In this case, a heater, the most likely fault would be one side of the middle of that heater faulting to ground. If you left it floating, neither secondary pole earthed, then there would be no problem at first fault, 
 If the heater is happy with one side earthed then apart from wondering if the transformer is needed at all (voltage change perhaps) the you can treat it like a low current TN-s supply and fuse the non earthed secondary pole. 
 If the transformer already contains a thermal fuse or similar then you do not need a secondary fuse to protect the transformer itself, and you may not need one at all unlessf you have multiple secondary loads and wish to protect the others from a failure of one. 
 A very small transformer, and you do not mention wattage, may not be able to muster enough secondary current to operate a trip or fuse sized to survive the inrush,- but a look at the secondary side voltage drop on load will tell you what the short circuit current would be. 
 I'm sorry if it is more questions than an answer, but with a bit more info, it may be possible to zoom in on a solution. 
 Mike

Isolation Transformer and its Secondary Protection

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