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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!
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  • Former Community Member
    0 Former Community Member
    mapj1:

    looking at the photo of that open lead, I think we have to imagine the chip 'doing the splits' between two things at not quite the same voltage,  both nominally earthed at the mains socket, but via quite a large loop of cables that gives some inductance and a time delay to the two paths and a fast spike may get round one way or the other such that there is a short duration large voltage difference at the chip. 

    It is the sort of design that keeps EMC consultants in business, no series chokes, no clear marshalling of the screens, no overall screening can - lines from the chip direct to touchable pins, with no obvious series current limiting choke or ESD trapping - false economy all round..


    The two fixes are to either reduce the area of the pick up loop, (shorter fatter leads or an over braid on the lead between PC and monitor), and/or  better to stop the spikes getting in there  in the first place - which may or may not be mains borne - the swap-over test will tell..

    Mike

     


    We've just had the monitor go off so swapping the phase has not worked. Just can't get my head around this at the minute as the more I look at it the less it makes any sense! Definitely missing something here!


    People who have had the same leads from the old office set up, and just moved them to their new set up have had the same issues as people who have had new leads. People with the same monitors and PC's as before they moved, all having these issues, people on different phases! Rings tested off ok (R1R2 and ins res) lamps switching monitors off, even when on different phases to one another, Scope's showing crazy readings, deviations on the phase waveform, and the noise on the monitors themselves etc etc.. either someone has given me false info on the leads and they are all faulty / temperamental or there's something very very bizarre going on. I've tried so many different leads though.


    It's just a normal office set up mirrored exactly the same on both sides and shouldn't really need any special leads for monitors as far as I can tell.


    I understand that scope could lead me on a wild goose chase but the reaction to the lamp on the problem side of the room is very real and you do see the difference on the scope.


    I'm going to do an earth loop impedance test tomorrow and then after that i'm going to go back in time 24 years, and continue with my carpentry and joinery because I never had a stair case just stop working all of a sudden!


    Oh, and there's someone in the purchasing dept claiming that her monitor keeps going off and that she has replaced leads too .... her sockets are fed from DB8 (our problem ring is from DB8) and it's an old PC station that someone used before she moved up there without any issue!..... Maybe she has bad leads too I don't know anymore!


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  • Former Community Member
    0 Former Community Member
    mapj1:

    looking at the photo of that open lead, I think we have to imagine the chip 'doing the splits' between two things at not quite the same voltage,  both nominally earthed at the mains socket, but via quite a large loop of cables that gives some inductance and a time delay to the two paths and a fast spike may get round one way or the other such that there is a short duration large voltage difference at the chip. 

    It is the sort of design that keeps EMC consultants in business, no series chokes, no clear marshalling of the screens, no overall screening can - lines from the chip direct to touchable pins, with no obvious series current limiting choke or ESD trapping - false economy all round..


    The two fixes are to either reduce the area of the pick up loop, (shorter fatter leads or an over braid on the lead between PC and monitor), and/or  better to stop the spikes getting in there  in the first place - which may or may not be mains borne - the swap-over test will tell..

    Mike

     


    We've just had the monitor go off so swapping the phase has not worked. Just can't get my head around this at the minute as the more I look at it the less it makes any sense! Definitely missing something here!


    People who have had the same leads from the old office set up, and just moved them to their new set up have had the same issues as people who have had new leads. People with the same monitors and PC's as before they moved, all having these issues, people on different phases! Rings tested off ok (R1R2 and ins res) lamps switching monitors off, even when on different phases to one another, Scope's showing crazy readings, deviations on the phase waveform, and the noise on the monitors themselves etc etc.. either someone has given me false info on the leads and they are all faulty / temperamental or there's something very very bizarre going on. I've tried so many different leads though.


    It's just a normal office set up mirrored exactly the same on both sides and shouldn't really need any special leads for monitors as far as I can tell.


    I understand that scope could lead me on a wild goose chase but the reaction to the lamp on the problem side of the room is very real and you do see the difference on the scope.


    I'm going to do an earth loop impedance test tomorrow and then after that i'm going to go back in time 24 years, and continue with my carpentry and joinery because I never had a stair case just stop working all of a sudden!


    Oh, and there's someone in the purchasing dept claiming that her monitor keeps going off and that she has replaced leads too .... her sockets are fed from DB8 (our problem ring is from DB8) and it's an old PC station that someone used before she moved up there without any issue!..... Maybe she has bad leads too I don't know anymore!


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