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Thread: Low Voltage Warning

  1. #1
    Diamond Member AndyD's Avatar
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    Low Voltage Warning

    I've had a couple of issues recently with unexplained customer systems failures. We've had one system that was intermittently dropping its RS485 comms network and on a completely different installation we've had 2 failures of an actual PLC within 2 months.

    With the comms failures we went through the usual proceedures of checking the integrety of the power, the earthing, bonding and network shielding arrangements as well as checking the clamping voltages and effective operation of the surge arrestors that were installed. Nothing came back as problematic but 3 network cards later I put a power quality analyzer on the system for a 5 day soak test. The first and most glaring problem that showed up was that the supply voltage after load shedding incidents was as low as 176V immediately after it was restored. 15 minutes after the power was restored it was still hovering around the 195 volt mark and it was a full hour and a half before it was stable at around 118v and over 2 1/2 hours before it approached the 'normal' supply voltage for that particular installation which is 238v (they are the closest customer to the Eskom MV transformer).

    I installed the same PQA on the PLC control panel at a different customer and it also showed a similar low voltage issue for an equally long period after a load shedding incident. Both of these systems have now had their power supply units replaced with alternatives that have a wider tolerances of input voltage and, touch wood, both issues seem to have been resolved.
    Last edited by AndyD; 15-Apr-15 at 02:40 PM.
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  3. #2
    Diamond Member AndyD's Avatar
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    Another problem that's far more important than these outlined above is one I've been having with my internet router at home. I have a decent Netgear router which has been giving intermittant problems with ADSL connectivity. I had my ISP check the system from their side and I've had Telkom out twice who ran several line tests then changed hardware and remapped my port on their side.

    The problem still persisted so I replaced the POTS filter and connecting cables as a matter of course and I changed my router another identical one I was given by a friend that emigrated and I had kept as a spare. Still the same issue persisted and by now I was convinced it was a line problem because everything on my side had been replaced.

    After several more failures I eventually noticed that there was nearly always connectivity problems that started about half an hour after the power was restored after we'd had a load shedding period. I put 2 and 2 together and I bench tested the router power supply adaptor with an input voltage of 200v. Sure enough the 12VDC output was at < 10volts with a simulated 75% of it's full load ability. I bought a new 12vdc power supply with an input voltage range of 100-250vac and problem solved.
    Last edited by AndyD; 15-Apr-15 at 02:41 PM.
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    Diamond Member AndyD's Avatar
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    So for anyone reading this who has unexplained occurances with electronic devices, I'd suggest bear in mind the load shedding can present problems even after the power comes back on. If you have devices with the old wire-wound transformer power supplies they'll be especially prone to low voltage issues and even some electronic type power supplies have a fairly tight voltage requirement (220-240v). Hopefully my experiences may save some of you the expense of replacing entire items when a new power supply unit may resolve the problems.

    I'd be interested to know much damage this is doing in general. I've replaced 2 other power adaptors on other electronic gadgets which were rated at 220-240v input requirement as a precaution. It's not so easy with whitegoods such as washing machine and microwaves which also contain sensitive electronics. I guess it would be prudent practice to avoid using any of these for a couple of hours after the load shedding is over.
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    Diamond Member AndyD's Avatar
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    Another thought that occurs to me is if you're considering installing an inverter as a back-up for your office equipment it would be worth spending the extra on one that you can set the minimum acceptable supply voltage at 210 or thereabouts. That way it will remain in battery mode after the power is restored and only return to normal supply mode once the voltage has stabilised. The problem this presents from the design side of a back-up system is you need extra battery capacity to cover this extended back-up period. The duration is something of an unknown though.

    Looking at the power logs from my last two load shedding episodes (both actual outages were 2 hrs 15 mins), the back-up time requirement until the voltage was consistenty above 210v would have been 3hrs 32mins for the first one and 4hrs 8mins for the latset one yesterday evening. Realistically this would be double the battery capacity requirement that you'd require for the actual outage .

    Even a standard UPS would require similar extra capacity
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    Gold Member irneb's Avatar
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    I've had this situation for the past few years in JHB. Even the geyser's element "blew" due to these poor quality power situations after the Eksdom power came back on. Never mind any electronic equipment, I've had 2 sound systems die horrible deaths, and I'm now on my 3rd UPS to my PCs for this year (yes less than 4 months on 3 UPSs).

    It's not so much the power outages, it's the dips and spikes (as you've also found) which kills anything electric. Will have to start looking at a house-wide inverter and voltage regulator, forget the genny idea - I just don't want to be replacing equipment at the rate I've been going. Either that, or disconnect Eish-donno totally ... just that proper gennies / other means are excessively expensive.

    On a side note, my cousin's in Kenya. He says that situation is "normal" over there. Everyone's got voltage regulators on every single appliance (i.e. at least a pass-through plug, more often than not built-into the house's circuitry) since their rule of thumb is not so much black outs as brown outs with huge spikes thereafter. Nearly everyone's got their back-yard genny with some even on full fledged UPS backups for stuff like normal kitchen appliances. Seems this is becoming par for the course with our useless crowd of anti-power suppliers over here too!
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    Low Voltage Warning

    Andy with all you knowledge and experience i must say i am surprised to hear you dont have pure wave online UPS units backing up important electronic equipment.

    A word of advice to anyone who has critical electronic devices or even a network with a pile of computers. There are a few essential requirements regardlesss if there is load shedding or not. The power supply in Durban for example is so unstable even on a good day. You need to take precautions.

    Lightning/ surge protection at entry to the property
    Trip connect
    Surge/lightning protection at the main DB
    Pure sign wave "online" ups supplying all sensitive electronics.

    20 years ago i had a company complaining that they were loosing Data and the network wasnt saving information. It got so bad that they ended up contracting the services of an external company to save info on the network. I fitted an online ups back then , 20 years later they still dont have problems with data loss or hard drive failure.

    It seems people are stupid when it comes to electronics...hard drive fails just buy new one, fails again , waste thousands on a server with a big hard drive or buy 2 hard drive and backup on both.
    If you stupid enough to only spend 0.1 % of your electronic equipment budget of a backup system then expect constant failures in your system and i am not talking about power supplies i am talking loss of data on hard drives, blown lamps on large printing machines, unexplained intermittent faults in general.

    A cheap R 600 UPS is gona do nothing for your system. Spend the money on a decent online system or suffer the consequences. It is that simple.

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    Gold Member irneb's Avatar
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    Quote Originally Posted by ians View Post
    A cheap R 600 UPS is gona do nothing for your system. Spend the money on a decent online system or suffer the consequences. It is that simple.
    Agreed with the rest you'r saying. Not that I disagree with this last line, just that I'd like to know what you'd suggest for the home? I.e. a handful (at most) PCs/Laptops, a couple of TVs, DVD/BR, sound system, etc. What sort of cost would an Online UPS with (say) around 5kVA (minimum - probably keep some stuff running for around 15min after failure so I can properly shut them down) backup be? Also what sort of circuitry changes would need to happen - i.e. if a central UPS, that would mean a "clean power" circuit in addition to the normal "dirty" power.

    For a business this makes a lot of sense, for a house not so much - unless it's a new house and laying both circuits isn't going to be "that" much more expensive (while they're at it anyway).
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    For a pure sine wave inverter with a 50Amp 24V battery charger - a 5KVA will cost about R21K ex VAT.
    You must still add the batteries.
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    As i mentioned "critical" electronic devices. In some domestic dwellings, the owners Tv cost 3 times the cost of a 5 kva inverter, thats just the TV. A smart house electronics go into the hundreds of thousands. Back to my comment 10 % for protection and another 10 % for backup wouldnt seem unreasonable.

    I went to the dentist a while back and he was showing off all his new fancy hi tech equipment. He hinted that the equipment cost him over R200 k. He asked me why I was smiling, i pointed to his 2 bob UPS under the desk. He said he didnt have the extra money, but did have good insurance. We wonder why insurance companies have to recover so much from our premiums and have have some much small writing in the contracts.

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