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Voltage Monitoring Relays - what delay?

AJJewsbury

There's been some talk recently of using voltage monitoring relays (e.g. https://uk.rs-online.com/web/p/monitoring-relays/1026131 or https://uk.rs-online.com/web/p/monitoring-relays/0122215), and a suitable contactor, to protect an installation against long duration overvoltages (so not brief surges, more like a single phase installation being fed something approaching 400V due to a broken N in the 3-phase distribution system).

I can see pros and cons to such an approach, but let's go with it for now for the sake of debate...

What I've noticed that all these devices seem to come with a programmable delay (sometimes overridden for large voltage errors, sometimes apparently not) which implies that the installation may have to withstand an overvoltage for some period of time - so the question is how long would we expect things to survive? I can see simple resistive heaters hanging on for a a fair fraction of a minute before overheating becomes catastrophic, filament lamps I suspect won't last anything like as long. What about electronics? or small single phase motors?

I suspect we're treading a fine line here, too short a delay and it'll be tripping out on the slightest glitch (next door's storage heaters switching on, or I suppose these days heat pumps, or a brief fault in some other installation connected to the same distribution system) which wouldn't be ideal even if the installation is only disconnected for a few seconds before being automatically restored. Too long and I presume the risk of damage increases. Where's the happy medium (is there one?)

  - Andy.

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  • As a matter of interest are these sort if over voltages really a big issue especially as now virtually every time the DNO dig up a duff joint for repair they earth the neutral with a copper rod or length of wire I've seen it done round here on repairs once when they put in a new linky box  and once when they put in some new joints  only the one with  feed to customer was earthed the other thru joints didn't seem to be

  • in reply to Kelly Marie Angel

    Well, one of the reasons they add NE links and extra electrodes when they dig up blown joints is exactly because of this sort of problem. Not sure what cable you have round your way but here we have a lot of copper singles overhead providing TT that is very reliable but pre-war, so a bit undersized for modern demand,  and on the postwar housing estates some terrible underground 1960s and 1970s Aluminium clad stuff that presents as either TNS or  TNCS at the customer service head,

    Where the plastic jacket has been scraped either at installation or by tree roots or settlement over time, the aluminium gets damp, and then over some years converts to the oxide as an insulating white encrustation, and at that point, depending which bit of the network you are on,  just the earth or the PEN is lost. We seem to have a few of those events a year but not all lead to full 400V on the live, it depends on the loading at the time it finally goes open circuit.  

    The DNO seem to be well aware of the issues as repair teams  turn up very fast to those ones.

    Mike.

  • in reply to Kelly Marie Angel

    now virtually every time the DNO dig up a duff joint for repair they earth the neutral with a copper rod or length of wire

    Individual additional electrodes make only a small difference as the resistance of the soil around the rod/wire is often into the tens if not hundreds of Ohms while the remaining connections and/or loads, which are pulling the other way as it were, are often in the Ohms or small fraction of an Ohm.

    If you're lucky and have many electrodes on your side of the break, your earthing system may well be kept closer to true Earth, but often at the expense of the transformer's star point drifting further way - which skews the L voltages (compared with Earth) and your L-N voltages are still liable to be outside of the normal range. There's no easy fix for these sorts of things.

       - Andy.

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  • in reply to Kelly Marie Angel

    now virtually every time the DNO dig up a duff joint for repair they earth the neutral with a copper rod or length of wire

    Individual additional electrodes make only a small difference as the resistance of the soil around the rod/wire is often into the tens if not hundreds of Ohms while the remaining connections and/or loads, which are pulling the other way as it were, are often in the Ohms or small fraction of an Ohm.

    If you're lucky and have many electrodes on your side of the break, your earthing system may well be kept closer to true Earth, but often at the expense of the transformer's star point drifting further way - which skews the L voltages (compared with Earth) and your L-N voltages are still liable to be outside of the normal range. There's no easy fix for these sorts of things.

       - Andy.

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