Coordination of insulation monitoring devices

This is really rather a difficult problem, because you are presumably using equipment not designed to work together. Can you give more detail on the application, because the usual way to have multiple circuits with individual devices is multiple isolating transformers or windings. There is also a problem with stray capacitance if the circuits are too long or connected to many items of equipment, although this is not usually met with hospital applications. I don't really see why an appliance should have the IM system, perhaps you can enlighten us?

PV central inverters are the classic that comes to mind, with an IMD primarily looking into the DC from the AC side (but also seeing the AC). Normally connected directly to the transformer IT0N, all good. But sometimes one comes across cases where there's interposing switchgear, perhaps to supply auxiliary equipment and controls (normally via isolation transfomrers) meaning that you can switch off the protection to the upstream circuit(s)... Not uncommon if the maintenance contractor wants the lights on to service the equipment, and may concievably persist for a while if there's an equipment fault.

Yes, one could look at other protective measures, but that's not the question. Similarly thinking about more complex arrangements where old-skool "No fault zones" are not sufficient.

Other variations on this theme exist.

What sort of situation would have a grid-tied PV inverter connected to an IT system? Usually IT earthing systems are provided for small safety-critical systems (as they keep working on 1st fault) - e.g. railway signalling systems or critical areas of hospitals - so it would seem sensible to exclude any unnecessary equipment from that system (as it could only reduce the reliability of the system by increasing the possibility of a fault). Given the useful output from a PV system is always going to be variable, so can't be relied upon to supply critical systems, it would seem logical to connect PV systems further upstream.


Or are we taking about unusual situation where the entire installation is IT?


   - Andy.

Not that unusual to me: Large inverters (>500kWish) are almost invariably arranged IT and fed from a dedicated transformer. I believe the reason is to minimise hazard to personnel and risk of fire in the event of first fault, since sites are normally unmanned in a stressful environment, plus the international market they sell and customer preferene to avoid downtime (although I've never actually had a straight answer from a manufacturer).


Anyway, the feeling seems to be, as I first thought, that it's not possible to have multiple IMDs, and that where it appears necessary the answer is to either ensure that only one is "live" at any time (in some fail-safe manner) or to re-arrange things so it's not required.


Thanks for your thoughts.

I suspect that unless the makers do some sort of special kit to allow the systems to be interlinked, they will just report  each other's measurement currents as faults. (For clarity, I'm imaging systems of the sort   explained here   ) Certainly the simple DC injecting systems will interact.


 I'm much less less sure about the PRBS  type (PRBS explained  the "AMP" system is an example) I imagine these could in principle  be programmed with individual  Gold codes  (Robert Gold's low cross-correlating codes) to not see each others waveforms, but I'm not sure the systems are really all that sophisticated, and it makes for a slow system as you have to average the signals over several full code sequences to get the benefit of  discrimination between your own code and that of another system. (however it works OK in 3G phones, where all the signals are overlaid but each one has a unique unscrambling key)


If it does not exist, then inventing that will be hard , where there might be more mileage is with one detection node but with variations in what sequence of  actions is taken  at first fault - you could have an automated system that mimics an operator taking various branches out of circuit, to see which clears the fault condition, so in short order the minimal zone around the faulty section is turned off, and the rest keeps running.


If you really have to have nested systems, I think you'' need the manufacturers on side to tell you how or even if it can be done.

Mike.

The only time I've coordinated IMD's is where downstream devices are connected to loads that are "dead" - eg a large motor, where you monitor insulation resistance whilst the motor is off, but when it's on, you revert to the source IMD.


You can have IMD's on each source (where you have a series of IT Islands) - but not usually multiple IMD's cascades on a single source


Where the source feeds a number of feeders you usually have a single IMD and then a series of fault locators that manage each circuit - all usually operating from the same low frequency AC or DC  injection by the IMD


I think your initial assumptions are correct - if you are familiar with Schneider gear, then something like an IM 400 at source and an XD 312 for the feeders. Larger systems might use something like a central supervisor with a IMD per feeder connected to a transformer switchboard  (with or without circuit monitoring)


For Andy, there are some very large IT systems to be found where downtime or safety or both are paramount - I could point you towards IT distribution that covers some pretty large industrial facilities - dockyards are one example, large chemical plants another.


Regards


OMS