Three Phase 230V three wire systems

If I have correctly understood your description, it sounds a poorly designed system.

An isolating transformer for the control circuit is almost certainly a prudent addition. Operation of an emergency stop or similar facility needs to be reliable, even under fault conditions. A relay or contactor that sometimes stays in is not acceptable.

Isolating transformers are widely used for control circuits and I am not aware of any prohibition.

If the supply starts out as a 3 phase 4 wire system, but with the neutral not distributed to the point of use, then 230 volts between phases will be 133 volts to earth, not 110 volts.

If the system is a true floating three phase system without an earthed neutral at the origin, then with 230 volts between phases, the voltage between any phase and earth can vary between the full 230 volts and zero. Variable according to leakage currents and earth faults elsewhere in the system.

In either case, an isolating transformer will ensure a consistent voltage for the control circuit without any concerns as to voltages between phases and earth.

I must thank you for confirming exactly my thoughts. May i ask if you are a Chartered Engineer as i would like to present what you have said to my Engineering team. If you wouldn't mind can i copy/paste your reply obviously without your name to my management?

You are welcome to quote my reply and/or to forward it, it was made in a PUBLIC forum.

Do not however take my remarks as being utterly correct until confirmed by at least one other respected member. I believe that I have sufficient qualification and experience to make the suggestion that I have made.

I do not publish details of full name and qualifications, partly for fear of legal action if it all goes wrong, perhaps as a result of me misunderstanding the original brief description. Or even that description being inadvertently  misleading.

Returning to the actual control circuit, I would add an isolating transformer, in line with accepted practice. If one side of the control transformer is earthed, then consider carefully the consequences of an earth fault AT ANY POINT in the circuit. It is sometimes preferable for a control circuit to be "floating" that is without either side earthed. 110 volts is popular, but other voltages are used.

Fully understood and thank you for the reply. I look forward to some more input into this subject. 

I have taken into account the consequences and my risk assessment process was based on all parts in the system are plastic and non conductive at all, primary and secondary of transformer are protected by suitable rated MCB's and all E-Stops are Ex rated Exde so i feel we are ok to proceed.

I feel the benefits far outweigh the dangers on this occasion. I would prefer to completely re-design the control system but due to operational and internal politics it is far easier to temporarily fit the transformer and work on a more permanent fix.

At least the system is operational and will give us confidence it will work in a emergency situation, at present we don't have a shutdown facility.

I think I  agree with broadgage it sounds like an odd system but is there any chance of a sketch of a simplified circuit  or at least the key bits of it ? 

Clearly any emergency stop design that does not always stop is unfit for purpose (well yes !), but I'd like to understand better what is going on.
One end of a coil being live does not mean it is 'on'  if the other end is open circuit, just that it is floating live,  but of course any path to ground even if unintended means that some current will flow. Yes transformers are allowed but you may have to think about how things fail to safe - for this reason most EM switches are N.C as a power cut , or a broken cable also look like the kill signal.

Anyway, an interesting one.

regards Mike

(consultant engineer, but I'd need the buy in of my employer to comment professionally)

the relays always have a minimum of 90v on L2 side of the coil so it is intermittently not opening

Where exactly is this 90V coming from and how is it measured? If it's supposedly 230/√3 to Earth from being connected to a different line in lieu of neutral I doubt it's of itself the cause relay failing to release. If it's what's across the coil when it's meant to be de-energised though, that's much more interesting - and might (one of several possibilities) suggest high capacitive coupling on the switching side (e.g. extremely long cable runs) which together with a high impedance coil might suggest a more fundamental problem that no amount of sticking the supply through a transformer would address.

   - Andy.

Ok guys, thank you all for the input.

Without leaving myself open and revealing too much private info its a 3phase no neutral 230V supply to DB then single phase 230v feeding the DB for the ESD system. 130V to Earth each phase (no neutral). 6 E-Stops for full engine shut down system all series up so any one opens it opens the circuit and should in theory allow the relay to drop out and stop the engines, fail safe in theory. I would of liked to have seen a double E-Stop circuit so its a positive open when activated as both legs drop out and you dont get false shutdowns should one leg fail but it is not and only one leg so even a loose connection should in theory shut the engines down, again not a good design.

A couple of the cable runs are very long as AJ eluded to but we have disconnected all circuits, proved they not showing any earth faults (all above 500Mohms) and all have expected resistance when E-Stops are closed (in normal position) and all go open circuit when opened (around 1.5Kohms on a couple of the longer runs). They are all wired directly back to ESD panel and then series up inside the panel. When we activate any of the E-Stops intermittently 40% of the time the relays fail to open and sometimes try but chatter. This is not acceptable as we all know. Without completely redesigning the system i thought a quick fix to get the system back working until we go to engineering to redesign with safety relays etc etc would be to instal the isolation TFMR on the supply and by tying the L2 side to Earth would give us the 0V on L2 and 230V which will be 0V when E-Stops are activated. This would absolutely guarantee the system will work as it should and give us time to re-design the system to a SIL level 2 or 3 as it should have been in the first place.

It is a bit awkward as i am trying to get this fixed so not to compromise safety of our systems but as its a safety system we need engineering approval and thats the sticking point as they will not admit the design flaw. I have contacted Schneider who have agreed the relays may not operate efficiently with this type of supply so i think this is a good temporary option. The relays for information are just normal Schneider general purpose relays, they are not even safety relays which in my opinion is very sloppy design.

For information i have tested this before suggesting the fix and it worked as i though but i needed to remove it as it was not approved. My only concern was maybe the L2 to earth but as its isolated and no exposed metal parts at all in the circuit i risk assessed and deemed it acceptable to allow the system to work correctly again.

We could install bleeder resistors i suppose but again that would be a modification of the system and need approval so i was hoping to completely eliminate it with theTFMR.

I think we all agree it is a very strange one and one i have not come across before. It has been a good fault finding journey and a good learning curve also.

I'm afraid I do not understand the design, nor am I sure that anyone else can either! Please post a circuit diagram as I think you description is inadequate to make any useful comment.

David CEng

David, may i message it to you directly as i don't want to post drawings or delicate information on to this forum as it may put my position at risk.

A couple of the cable runs are very long as AJ eluded to but we have disconnected all circuits, proved they not showing any earth faults (all above 500Mohms) and all have expected resistance when E-Stops are closed (in normal position) and all go open circuit when opened (around 1.5Kohms on a couple of the longer runs).

If they're 1.5kΩ for a closed loop they must be a heck of a length or minute c.s.a. -  presuming the former I'd definitely consider the effect of capacitive coupling. As it's a purely a.c. effect it won't show up on a d.c. insulation test, but when live it'll be like having poor insulation between the switch wires where they run close and parallel. We often see things like LED lighting flickering when switched off just due to a few metres of T&E cable to the switch - if we're looking at perhaps hundreds of metres of cabling, the resulting current could well be significant.

If you could give us an idea of the length (and possibly type) of cable involved, we might be able to guesstimate the likely capacitance, and therefore current leakage at 50Hz. (I seem to recall that Mike had a rule of thumb pF/m...)

   - Andy.