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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
    How practical, or not, would it be to bring a supply in via another route - not quite at the level of an extension lead out the window to next door, but back from somewhere nearer the origin. I appreciate that may not be easy, but cables through the airlock door was OK for the MIR space station, so perhaps a test with a supply brought in may be worth it.

    Failing that a clean supply may be possible by filtering  (example of the sort  of thing I mean ) The idea is dirty in one side, clean out the other, and could be in a box beside the DB,  or indeed for debugging perhaps a lead with one of these and a plug on one side and 4 way socket on the other end (and adaptable box and some skintop glands if used as a portable test thingy.).


    If it  is not mains borne but is ESD induced, then yes, perspex screens and unearthed metal frames seem a recipe for possible trouble with static zaps, but I'd expect both sides of the room to be equally affected unless there is some odd source of extra humidity on one side

    Mike.




  • Former Community Member
    0 Former Community Member
    mapj1:

    How practical, or not, would it be to bring a supply in via another route - not quite at the level of an extension lead out the window to next door, but back from somewhere nearer the origin. I appreciate that may not be easy, but cables through the airlock door was OK for the MIR space station, so perhaps a test with a supply brought in may be worth it.

    Failing that a clean supply may be possible by filtering  (example of the sort  of thing I mean ) The idea is dirty in one side, clean out the other, and could be in a box beside the DB,  or indeed for debugging perhaps a lead with one of these and a plug on one side and 4 way socket on the other end (and adaptable box and some skintop glands if used as a portable test thingy.).


    If it  is not mains borne but is ESD induced, then yes, perspex screens and unearthed metal frames seem a recipe for possible trouble with static zaps, but I'd expect both sides of the room to be equally affected unless there is some odd source of extra humidity on one side

    Mike.




     


    I'm coming in early tomorrow again, before the office starts to fill and i'm going to try unplugging each none passive lead (They seem to have one passive and one none passive due to lack of correct connections on the back of the monitors) and then i'm going to see if the problem still exists both on the scope and in reality (Monitors switching off) After that, it's time to put both rings on one phase and see what happens for the day!


  • 0
    Could it be that the monitors on the affected ring are on a different phase to the main computer in the server room?

    It's not usual to have monitor leads going all the way back to the server room these days - normally the monitors on a desk are connected to a desktop PC or laptop on the same desk and then the connection to the server is via some network system - e.g. Ethernet. At least that's my assumption - so please someone shout if this one is different.


    Modern networking systems (e.g. twisted pair Ethernet) have balanced signalling so are pretty immune to differences in Earth potential between the ends.


    In the old days there might be terminals on the desk with RS-232 going back to the server room - which could occasionally be a challenge as RS-232 used Earth as a signal reference.


    Things on different phases usually isn't a worry - since all the equipment runs on d.c. internally. In fact it's perfectly normal to have a 3-phase supply to a server rack with adjacent equipment or even the two power supplies of a single item of equipment fed from different phases. Some even have twin supplies (for redundancy) with at least one fed by a UPS which can sometimes be entirely out of phase with the mains supply - so you can in effect have six different 'phases' all jostling around in the same area - all perfectly normal.


       - Andy.
  • Former Community Member
    0 Former Community Member
    AJJewsbury:
    Could it be that the monitors on the affected ring are on a different phase to the main computer in the server room?

    It's not usual to have monitor leads going all the way back to the server room these days - normally the monitors on a desk are connected to a desktop PC or laptop on the same desk and then the connection to the server is via some network system - e.g. Ethernet. At least that's my assumption - so please someone shout if this one is different.


    Modern networking systems (e.g. twisted pair Ethernet) have balanced signalling so are pretty immune to differences in Earth potential between the ends.


    In the old days there might be terminals on the desk with RS-232 going back to the server room - which could occasionally be a challenge as RS-232 used Earth as a signal reference.


    Things on different phases usually isn't a worry - since all the equipment runs on d.c. internally. In fact it's perfectly normal to have a 3-phase supply to a server rack with adjacent equipment or even the two power supplies of a single item of equipment fed from different phases. Some even have twin supplies (for redundancy) with at least one fed by a UPS which can sometimes be entirely out of phase with the mains supply - so you can in effect have six different 'phases' all jostling around in the same area - all perfectly normal.


       - Andy.


    Morning,


    Yes, you are correct, this is just a standard modern day set up with monitor leads plugged in to local PC's under the desks. 


    Dave


  • Former Community Member
    0 Former Community Member
    Morning update.... Yesterday after taking the monitors off the steelwork after 2 damaged leads in quick succession on the same user, I thought I was getting somewhere as she had no problems for the rest of the day... right until the end of play when both hers and the person next to her monitors went off and back on again. Frustrating!

    I have now swapped the phases over this morning to see if the problem moves over to the other side.


    It is definitely only a problem with the none passive leads. There is 4 ohm's resistance end to end straight out the bag measuring the outer of each connector (no continuity at all after they fail). This obviously get's plugged in to the PC and the monitor which are both then earthed via there power leads. The monitors are then screwed to "extraneous" steel shrouds that become exposed extensions of the earth via the monitor earth in the power lead... so I thought I might be on the something by taking them off the steel work but no, not the case.I have only removed one person's monitors though so unless something strange is happening cumulative across the other monitors.


    I'll see what happens today now L1 and L2 rings have been swapped over.


  • 0
    Hi Dave


    The symptom is therefore a large current passes through the HDMI screen (which is very thin foil) and it fuses. HDMI with a disconnected screen is not at all reliable, and the fault could damage the Earth referenced power on the cable to the electronics. Somewhere above I think you said there were 4 Ohms between the two ring Earths, which is excessive and cannot be right, I think you just found the 4 Ohms, the bad ring has no Earth to the Steelwork and a significant Earth fault somewhere too. You should find this with a full ring test, and Earth loop measurements, along with testing to find the intermittently faulty piece of equipment. These currents could be circulating Earth current from elsewhere, a clip-on Ammeter will help with that.


    Regards

    David
  • Former Community Member
    0 Former Community Member
    davezawadi (David Stone):

    Hi Dave


    The symptom is therefore a large current passes through the HDMI screen (which is very thin foil) and it fuses. HDMI with a disconnected screen is not at all reliable, and the fault could damage the Earth referenced power on the cable to the electronics. Somewhere above I think you said there were 4 Ohms between the two ring Earths, which is excessive and cannot be right, I think you just found the 4 Ohms, the bad ring has no Earth to the Steelwork and a significant Earth fault somewhere too. You should find this with a full ring test, and Earth loop measurements, along with testing to find the intermittently faulty piece of equipment. These currents could be circulating Earth current from elsewhere, a clip-on Ammeter will help with that.


    Regards

    David


    Hi,


    Yes, someone previously mentioned about the 4 ohms being too high and I immediately realised i'd been a bit flippant about that reading as it is indeed high. I retested it but dropped the socket fronts off and got an improved reading of 1.5 ohms. Still high but maybe not alarm bell high!


    Here's a pic of one of the failed leads showing the chip and it's connections. No wires, including any shielding can bypass the chip by the looks of it...a9d1f22eadb2900519c44c5b22b66116-original-12.jpg


  • Former Community Member
    0 Former Community Member
    I'm just waiting to see what the outcome of swapping the phases over is today, as of yet there have been no 2flickers2 reported.
  • Former Community Member
    0 Former Community Member
    Sparky Dave:

    I'm just waiting to see what the outcome of swapping the phases over is today, as of yet there have been no 2flickers2 reported.


    "Flickers" ?


  • 0
    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

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