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PC Monitors switching off and DVI to HDMI cables failing

Former Community Member
Former Community Member
Hi,


I'm new to the Forum but i'll try and explain this as best I can as it's a strange one!.. Sorry it's a long one!


The issue -

For several weeks now, there have been reports of multiple Monitors intermittently switching off and on again across 5 or 6 different PC users in an office of 20 PC users. Sometimes as little as twice a day but up to as often as15 times a day Apparently, this has led to multiple monitor leads no longer working and being replaced but the issue still remains.


The system -

Sub DB fed from Main DB on a TNCS system (The office is part of a larger factory / workshop set up). The circuit in question is a 32A Ring with a 30mA RCD up front, feeding 24 twin sockets in 4.0mm2 T+E. It is dedicated to office use only. Only 1 of the 2 earth connection points have been used on each socket outlet and no extra earth has been ran alongside it, so simply a ring main earth and that's it. It is a fairly new installation but the equipment has all been used before as the personnel have just moved all their stuff from an old office into a new one. There is an identical ring circuit feeding the other side of the room with the same set up but with no issues with their monitors reported (same monitors and fed from the same Board).

For the most part, monitors and PC's are plugged directly into the wall sockets but there are a few stations that are fed from 4 way extension leads. Problems are occurring on both set ups.


What's been tried / observed already -

Socket fronts dropped off, no N/E reversals and all connections tight. R1R2 = 0.05 ohms and insulation resistance test at the board = >999 Mohms so no issues across any combination of conductors.

Mis-leading -

I had a cheap and cheerful meter connected via Bluetooth that was sampling and logging every second, that picked up frequency spikes of up to 350Hz! I am now convinced that this is a banana skin and that the meter may have been suffering some communication interference (maybe caused by the same fault dropping the monitors out?!) as I later connected an Oscilloscope which did not show any fluctuation on 50 Hz. Thought i'd mention though just in case!


What the scope did pick up though, was a voltage on the exposed metal casing at the bottom of the monitor at the rear (Where the DVI ports etc are) that spiked whenever the monitor was turned off via the fault.


Here's where it gets weird.... The PC users, prior to my involvement had set up a 100w lamp (Bayonet type) to see if it would flicker when the monitors turned off (which is doesn't) but in actual fact, when you turn the lamp on and off, it would throw huge spikes onto this metal casing at the back of the monitor! I measured up to 100V on the scope! It would also turn the monitor OFF/ON. The lamp has no earth connection as it is all plastic housing.

When I plugged the lamp into the sockets on the other side of the room (Separate ring circuit) it did make a slight reading on the scope when probing the same part of somebody else's monitor but nothing like the amplitude I was getting on the problem circuit. More like 5V rather than 100V and no monitor issues either.

I can only assume that the RCD isn't tripping because it is such a fast spike! However that does surprise me that it would still damage a lead!?

I have turned off some of the Aircon unit circuits for as long as i've been able to as I wondered if a faulty inverter may be throwing something down the earth but it didn't seem to reduce the flickering.


So yeah.... Help!
  • 0
    Welcome to the forum, and it seems also welcome to the wacky world of EMC . 

    You are not alone..

    You seem to be looking for earth loop type issues. That may or may not be the right path but let's bottom that out if we can.


    Some questions I'm afraid before any answers may or may not emerge from the fog, as I'm struggling a bit to visualise the set up.


    So, are these monitors class 1 (earthed mains lead) and if so does the mains earth also connect with the signal earth of the video cables internally to the monitor ?

    (just because there is a mains safety earth does not always mean that all the exposed metal bits of connectors are at that same voltage - indeed things like Ethernet very carefully do not do that. 

    (a multimeter of the cheap kind on it's ohms range that only uses a few volts is the way to test this, not the megger or anything capable of large and damaging currents please.)

    What else plugs into the computers - are there other cables that leave the zone, such as network, fax or telephone cables, or any other other weird and wonderful local wiring.  What else shares the dirty ring - anything with a water pipe connection or another external conductor like that, which might be an extraneous conductor and could be pulling the N-E volts about ?


    Are the computers class 1 (and if so once again does the mains earth get to the backshells of the video cables etc...)?


    If you run a wander lead from the earth of the clean ring main 1 to the  dirty ring main 2, is there a potential difference between them ?


    Your scope tests - how fast is that  fast pulse to the nearest order of magnitude , tens microsecond, a few nanosecond ?  It matters a lot for the correct fix.


    This 100V spike - relative to what ? it may just as well be that the monitor is staying still but the scope ground is bouncing - you would not be able to tell .. Normally we assume the ground is fixed and the signal moves but it is only a convention.


    Now the 100W lamp is interesting -  is that spike just on switch on ? (cold inrush, suggests a high Zs to high frequencies ) or just on switch off? (inductive back EMF, suggests a long supply cable ). Or both ?


    Do the good and bad rings share an origin, and if not, what are differences in their supply arrangement, and are they on the same phase ?


    Are the blown leads passive, or do they contain some clever electronic interface in the plastic bump at one end or the other ?


    To blow a passive lead suggests fuse-wire melting sort of energies, to cock up electronics however requires a few microjoules in the wrong place.



    Here's hoping for a killer clue...


    Mike.
  • 0
    And another one, if the DVI to HDMI conversion leads are active (have electronics ) they may well not provide a low resistance path between the backshells at the two ends, in which case you need monitor and PC on the  same socket ideally. Or for tests, all things on any one desk on a common 4 way block. so that it does not 'do the splits' between two pieces of kit that are not at the same voltage during the surprise events.

    Mike.
  • 0
    Hello SparkyDave.

    An interesting problem for us today! When you say the "monitors drop off" do you mean as though the on-off switch is pressed or just the computer fails to drive a picture to the monitor? The HDMI leads you say are faulty, are you sure because unless they are very long they are simple wires inside, 2 twisted pairs and a power and ground too? I will now ask what (if anything has failed because this is the best clue to the problem. I know the connectors are rather small and connecting to the pins needs a very fine needle probe, but a full set of continuity reading would be a great help. I forgot to say that there is an overall screen too, which should also be connected to a number of pins, but a simple pin to same pin each end is what we need.


    Have you tested the continuity of the earth lead to every computer and monitor to any exposed metalwork, HDMI sockets should be connected to Earth at both ends, because the connection plays no part in the signal transmission? I assume all the computers are networked, but this should not be a problem because the cables are completely isolated from Earth at both ends and such networks are pretty robust. Mike might be right about EMC, so I need to know about your oscilloscope. This will only respond to differences between the probe tip and the associated clip lead, where did you connect each. If the clip was floating I'm afraid the results you got are probably nothing too odd.


    Thanks for the answers

    Regards David
  • 0
    Not all monitor leads are just a wire with a plug on each end.  If the connectors are different at each end, then one of the plugs might be hiding electronics to convert between standards.  That could be fried by a voltage spike.


    Have you peoperly investigated the state of the ring circuit?  If a 100W lamp causes a voltage spike, then it suggests that something may be seriously wrong with it.
  • 0
    You mentioned voltage spikes and a workshop next door. When did this problem start what changed before it started?


    Sounds like a case for fitting surge protection if transients are spiking the IT kit. A Type 2 at the local DB and Type 3 surge protected extension leads for the kit to plug in to. Not the cheap rubbish from the pound shop decent ones from Masterplug or Belkin.
  • 0
    An example that is hopefully not your problem ...

    My point is that another apparently innocent thing that can cause odd tripping sort of problems is ESD from tribo-electricity (i.e. insulators rubbing - like the old balloon on a jumper sticks to the ceiling trick, but with more volts. ). Has there been a change of furniture or floor coverings recently, and is the effect weather dependant  ?

    Mike
  • 0
    My first thought with monitors suddenly going off would be a software issue - with the windows OS getting updates automatically over the internet these days (and the same for most other OSs), and compatibility problems with odd device drivers still not unknown, and all sort of things trying new ways of saving power these days, stranger things have happened.


    That wouldn't explain video leads going faulty of course (if indeed they are faulty, rather than a problem happening to go away when they're changed). Some nasty spikes would be a 2nd guess thought though.


      - Andy.
  • Former Community Member
    0 Former Community Member
    Sorry folks, II had to pause for the day as they kicked me out and went home! Who can blame them! I'll try and just respond here if that's ok and thanks for the suggestions!

    Ok, so the Monitors and the PC's are indeed Class 1 and the earth is continuous from the exposed metal parts on the Monitor / PC, through the DVI leads outer exposed parts and on to the socket earth terminal. All at the same potential. I have noticed 2V between neutral at the socket and the exposed metal on the Monitor.

    The Computers have Ethernet connections, so there are data cables running to each one from the server next door. This has so far remained unexplored. This problem did not exist for the first few months of the offices being used apparently but I can't find anything obvious that has changed.

    I can see nothing else that plugs into this ring other than the individual users PC's and Monitors etc and no sign of anything else. 

    There's no potential difference between the earths on either ring. 00.01V's and 4 ohms between the two using the wandering lead method.

    Regarding the leads, so far there have been 3 types that have become faulty and a total of about 15 altogether, VGA to DVI, DVI to HDMI and HDMI to HDMI! Infact, one has just gone this morning from flicking the 100W lamp on and off repeatedly. 

    Which leads me on to this morning's saga so far...Let me just briefly explain how the Scope is connected before I try and show you pics.The scope is a Keysight DSOX1102G. I have channel 1 probe connected to Line of the ring via a Choccy block and the probes earth clip connected to earth on the same Choccy block from the ring. channel 2 probe is connected to the metal casing at the back of the monitor where the DVI / HDMi leads plug in (Remember this is continuous to earth) and it's earth clip connected to the ring earth in the choccy block also.

    Now that's established i'll try to explain what i'm seeing. So when I turn on/off the 100W lamp (Spikes are bigger when turning the lamp on but also exist when turning off and both ways turns off the monitor) the flat green line on probe 2 starts to show flicks of varying amplitude on the screen. Some are quite fat (time) but even the fattest flick amounts to around 500 micro seconds! They range from around 30 to >100V. Here's where it's blown my mind!... After flicking the lamp on/off several times the scope showed that it had gone out of phase!? You can also see that the amplitude of the sine wave is also effected when switching off the lamp so this may indeed not just be an earth fault issue. Bare in mind that I have done this exact same test on the other ring main and none of this is happening.


    The scope captures any changes, almost like it leaves a stain on the screen if that makes sense? That is what you are seeing in the pics where I have flicked the lamp on/off many times. I also lowered the green line on one of the pics so I could see just how high they were spiking but strangely they were not that high on that occasion.


    Head scratcher!.....Bare with me as I can only load one image at a time for some reason37782373e8d2a8bc6d1053a164c32240-original-8.jpg
  • Former Community Member
    0 Former Community Member
    c4c32ae43516522cc2b2eb316d354f88-original-9.jpg
  • Former Community Member
    0 Former Community Member
    Hi,


    The IT team here have ruled out anything like that apparently but these readings suggest they are correct.


    Thanks

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