Chris, your isolation monitor sounds very comprehensive in what it measures. In this situation though, only the impedance measurement part is up for consideration of course. From the information you have given us, this is very definitely an equipment-derived (mains) line-to-earth impedance problem, unless you already have an isolation monitor/system problem (see testing method below).
If you have not already done it, even though such a low impedance (1K-ohms) is being registered when the video monitor is connected, suggesting the problem is almost certain to be fairly and squarely in the video monitor itself, I would suggest you do a quick check of the system integrity with respect to the overall ‘leakage’ of the system. Isolation systems can deteriorate unnoticed and should be checked on a regular basis anyway. This will give you to some extent ‘peace of mind’ that your isolation monitor is calibrated and doing its job properly, or not, as the case may be.
If you do not have an isolation system checker, then build yourself this quick piece of test kit I have outlined below. With this you can test for that 240K-ohms alarm-trip point you may still have your eye on adjusting! You can also measure the isolation leakage (current flowing) from both lines with respect to earth. (I will refer to ‘Lines’ now, not ‘live and neutral’ in the following explanation)
First off though, disconnect ALL the equipment in the theatre suite in question, from All the sockets in the wall/ceiling mounts etc. (Don’t forget wall-mounted viewing boxes, surgical angle lamps etc. etc. if plugged into the ordinary sockets i.e not hard-wired in). With this done, the isolation monitor should be reading a high impedance, say at least 4-500K-ohms? if the ‘trip-point’ of your isolation monitor is 240K. If the reading is still much lower, say around the 240 value, then you should attempt to find out why, as it might not take much to trip the alarm when extra equipment is plugged in later. Generally, isolation monitors are designed to alert the staff with a current limit of around 2 to 5mA. At your mains voltage and for this trip impedance (240k), this would equate to an alarm tripping point of just 1mA. (If it had not been showing as low as 1k-ohm when you plugged in your video monitor, I would have suggested you look for an accumulation of impedances that might have been just ‘tipping the balance’ over the 240K trip limit. But this reading is LOW!).
With the isolation monitor reading hopefully well above the 240K value and with all the sockets empty, I am assuming that if you plug in your video monitor now it will show approximately the same 1k reading and alarm - Yes?? If the reading stays low, and then returns very high when it is unplugged again before the alarm is reset, consider the video monitor as a very likely 'culprit’. Unplug the monitor again.
Now to test the system. Get a 240 volt light bulb connected to two wires and momentarily touch them across the two line (live & neutral) conductors in any mains wall socket to ensure it lights up. Then connect the two wires, one to a line conductor and the other to earth, in the same socket. It should not light up. Connect to the other line conductor and earth. In neither case should it light up. (If one line to earth does light, you do not have an isolated system any more i.e one side is already down to, or very near, earth potential. If this is so, stop here immediately and investigate the reason).
Note: The isolation monitor may alarm each time the bulb is connected from each line to earth, just reset and continue. Do not forget this light bulb check step.
If the light-bulb test gives the correct results, then next get an ammeter (it will be measuring only a low ac value) and measure the isolation leakage current (impedance-equivalent) at the same wall outlet, of each line to earth in turn. As you did with the light bulb, connect it between one line and earth (not between line and line in this test, of course!!) note the reading and then do the same between the other line and earth. Note that reading too. Now calculate the individual impedances from the individual current values of each line to earth. The highest current value (lowest impedance) reading should equate to the isolation monitor’s ‘estimated’ and displayed value of isolation impedance in the system. Note: for trouble-shooting purposes, the leakage current value you read in either line when doing this test, relates to the isolation value of the other line-to-earth and not the line-to-earth that you are measuring.
Now with the trip-point potentiometer tester you have built (see below), plug it into the same wall socket (or any other). Slowly reduce the pot resistance until the isolation monitor alarms on the line selected. When it alarms, note the impedance reading on the isolation monitor and then remove the mains plug and measure and note the resistance between the relative power pin on the plug and the earth pin on the plug. Now return the pot to maximum resistance and switch over to the other line. Plug it in again and do the same measurement and note the same two readings. Using these pot resistance readings you can firstly, with the help of dear Ohm’s law (and 240volts), simply work out the current that has tripped the alarm. It should of course be near the 1mA level in both cases. And then secondly, check that the impedance readings found on the isolation monitor compare favourably with the resistance readings on the pot. Assuming that the leakage current readings, you took above, were near enough the same, it should be approximately the same reading i.e trip the alarm around the same point on the pot. This will give you an indication of how accurate your isolation monitor is (in the impedance measuring mode only) and hence if you have a problem actually in the isolation monitor/system itself. The pot readings should be around the 240K-ohm mark if the isolation monitor is working correctly. (If not, it might also help to decide whether the pot you wanted to adjust really needs adjusting, though I would still recommend consulting the company first).
Now to return to the video monitor itself. As you said, you did the insulation impedance of both live and neutral together, to earth, on the video monitor and it passed OK. However, the main point to note here is that the Insulation Test is usually done with a DC voltage in testers, which might well leave undetected (as has already been suggested) say a ‘leaky’ interference-rejection capacitor (normally having mains-frequency voltage applied across it) in what in your case here (if the video monitor is not, like me, of the ‘old generation’) could easily be a switch-mode PSU.
I was going to suggest that you had possibly over-looked, due to perhaps limitations of your testers, doing the earth leakage test in the mains reverse connection. The reason being (for interest only) that in the normal mains configuration testing on a non-isolated supply, any caps etc. going to earth on the neutral side that might be leaky, would definitely not be detected due to lack of ‘electrical stress’ (normally produced by the live wire to earth) needed to produce a current and the earth-neutral link. But since you have done that particularly important test in this case, the two things that might be happening are firstly, that when you switch the video monitor on, the initial charge-up surge current (though very small) into the mains input filter caps (tied to earth) is causing the isolation monitor to momentarily trip but permanently latch the alarm - a condition the above system tests would be unable to reveal of course. (If the reading stays latched at 1k after straight away removing the video monitor from the mains socket, this might help to confirm the theory). If, however, the impedance reading does change back to high impedance when removing the video monitor whilst in the alarm condition, it might strongly suggest that you have a ‘hard’ fault to earth not a transient impedance dip. In this case it should be easier to find. The second possibility, and perhaps the more likely, (if it is an SM-PSU) is that one (or both?) of the MOV’s (metal oxide varistors) may be too sensitive (or too low a value) and momentarily triggering low impedance to earth at switch-on.
Now to the construction of the isolation monitor leakage alarm-trip tester:
Get yourself a linear (not log) potentiometer of say 470 kilo-ohms and a center-off, single-pole, double-throw mains switch. Connect a short piece of any size (a one or two-amp rating will do) three-core mains cable to the live, neutral and earth of a 13-amp plug (any rating fuse in it will do, as it will not be there for protection, only continuity) - or whatever type you use in theatres. Connect the slider of the pot to one of the other contacts on the pot as well as connecting a wire from this jumpered pair first to one end of an in-line fuse holder (with say a 100 or so mA fuse) and the other end of the fuse holder to the earth wire of the cable. Set the pot to read maximum ohms with respect to the other free terminal on the pot. Now connect this free terminal on the pot to the center pole on the switch. Connect now one power line wire in the cable to one of the two spare switch poles and the other power line in the cable to the other spare switch pole. Tape over any bare wires.
Note : Even though it is generally safer working on isolation systems, in certain circumstances (particularly across the two line conductors) it can still be dangerous, so use the usual amount of safety awareness when doing these tests.
Lastly for my own interest, did I interpret you correctly when you say that the isolation monitor is looking to measure dc current superimposed on the (ac) power circuit connected to the isolation transformer? Or did you in fact mean that it uses a superimposed dc to do the impedance measurements from the lines to earth ? The latter (if pulsed) to me would probably be more likely as it would need some form of a ‘changing’ current to measure not only purely resistive components to earth, but capacitive as well. If I am wrong, what dc is it measuring on the system? Measuring the temperature of the transformer in an isolation set-up was new to me as well. Interesting. Seems a very thorough monitoring device.
Finally, permit a note from an ‘old-hand’ on pot-adjusting. If the pot in question that you thought about adjusting is not easy to get at i.e. need to remove covers, it was good initial advice from the company. In fact as a basic technician tenet, it is never a good idea to adjust internal ‘pots’ in any piece of equipment as a way around problems, however tempting it may be, unless you have explored all the other avenues to try to correct the problem first. Although ‘pots’ can go noisy and upset readings (or be mis-adjusted by others), best wait, as one can easily end up with more problems than bargained for particularly in this case if you have to have the isolation monitor re-calibrated with specialized equipment later (at the hospital’s own expense?) having found the fault elsewhere.
If now you decide it is in the video monitor, (don’t rule out the small possibility that the isolation monitor may be reacting incorrectly to spurious switch-on dips), concentrate only on the PSU, (old style or smps) . If it is a switch mode, I would suggest (only to test of course) just removing the interference caps that go to earth and then MOV’s or vice versa.
Keep going.
