24/7 Unattended Comms Equipment - Preventing Electrical Fires?

[exotic]

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[ericmark]

Much depends on the design of building and personal access. Halon extinguishers also stop life as well as fires and where used there is a strict access code to ensure it is turned off before anyone enters.

It may be better to let items burn but for them to be on non inflammable surfaces. Simple floor tiles for example. Chinese imports are a problem where they do not conform with safety standards however they are cheap. Housing in an earthed metal shelving with correct fuse sizes will help.

If it were my building I would call on fire brigade for help they are the exports why not use them? Clearly using RCD protection will assist but it also means systems can trip and therefore not work.

 

[bernardgreen]

Avoid cheap ( and therefor often nasty ) equipment.

A PSU that will fail safe costs a bit more than one that can self destruct if it fails. Fail safe meaning that it is highly unlikely to catch fire before it disconnects from the mains ( internal fuding ) and even if the internals become extremely hot before the fuse operates the case does not burst into self sustaining fire.

 

[davelx]

Halon extinguishers also stop life as well as fires and where used there is a strict access code to ensure it is turned off before anyone enters.

AFAIIA, with a very few exceptions, fire supression systems using Halon have been banned since 31st DEC 2002 (including existing systems installed before that date) under EU Regulation 2037/2000. They should have been replaced with systems using more environmentally friendly alternatives - e.g. Argonite, Inergen, Proinert, which are oxygen displacing gasses (and therefore asphyxiants so still hazardous to people but not toxic). There are also chemical fire supressant gasses such as FM200 ( HFC227-ea) FE25 and Novec 1230, which are more environmentally friendly than Halon and less or non-toxic - FM200 is used as a propellant in medical inhalers! I remember all the Halon systems being replaced at Reuters Docklands Technical Centre when I was working there around that time.

In addition to the toxic or asphyxiant properties of the gasses used, another reason to ensure that the systems are disabled or set to delayed discharge when anyone is in the room is that the force of the discharge is quite capable of knocking people over, and/or propelling hard obects that could injure someone.

 

[ban-all-sheds]

What's the best way to prevent electrical fires being started by CCTV, PSUs, and other comms equipment which need to run 24/7 unattended?

The world is full of rooms, and entire buildings, housing unattended kit running 24x7. Yes, they do have automatic fire suppression systems, but these don't get deployed very often.

Prevention is better than cure - as Bernard says, start by stopping buying cheap and flaky equipment.

 

[endecotp]

it's not the first time I've had this type of incident with a PSU.

Unless you're managing huge numbers of devices, this sort of failure should be very rare. So I think your first step should be to replace all of your dodgy PSUs with reputable ones.

Your second step should be to make sure that you have a fire alarm.

I would be interested to see a photo of the label from the PSU. Does it claim to be UL listed, or similar?

 

[bernardgreen]

If you use PSUs that do not have an earth then the fault current will be that which the MCB can pass. So probably your compact PSU will be have aboput 240 watts of heat being created by the fault for several minutes before the MCB ( or wired fuse ) opens. That is going to get very hot.

A PSU with a good earthing arrangement under fault conditions may only have to carry 30 mA of fault current before an RCD operates. ( or 100 mA ) Either way the heat produced by the short duration fault current of 30 ( or 100 ) mA will be a lot less than the heat from a prolonged 1 amp fault current.

 

[endecotp]

A PSU with a good earthing arrangement under fault conditions may only have to carry 30 mA of fault current before an RCD operates.

I don't think the common failure modes of switch mode power supplies result in earth currents. What failure do you have in mind?

My understanding is that the most common failure modes start with electrolytic capacitors gradually failing (due to the electrolyte drying up). Often you'll notice this because the attached equipment will stop working due to e.g. excessive ripple. Eventually the control-loop is compromised due to the lack of filtering, and either the control IC will shut down the supply safely, or one of the semiconductors (power transistors or possibly diodes) will fail. If a power transistor fails short-circuit, ideally it will blow a fuse. If the power supply burns or melts, this suggests to me either no fuse, or too high value fuse. Or maybe a badly-chosen PTC thermistor fuse.

 

[Iggifer]

Depending on the lengths of your cable runs and how it's wired, you could always wire them back to one PSU located with your DVR, something like this might work for you, fused outlets, and a metal casing, which should help prevent a fire should something go wrong.

 

[AdrianUK]

The wallwarts that I use aren't plugged directly into a 13A socket.... they are in a short extension/4 way blocked which is protected by a 1A or 2A fuse.... hopefully this will provide less energy let thru in the event of a failure & result in a less violet death

 

[exotic]

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[ban-all-sheds]

Here's how it looked afterwards, with label visible:

So no symbols for UL, CSA etc. And no CE, as in Conformité Européenne, mark either. That PSU is illegal here - it should not have been on sale.

While I agree with the sentiments towards Chinese manufactured goods, the paradox is that almost all electrical items are manufactured in China nowadays.

Yup. And some of them are even legal, and safe, and have genuine CE marks, not fake ones like that one had.

£75 is a bit much for PSUs used in the home

And fires are a lot cheaper than that, too, aren't they.

the features and principles of the items in your link are ones that I can adopt without breaking the budget.

Yes - you do need to make sure you keep money back for replacing possessions, paying for funerals etc.

I already have this similar PSU that I never found a use for yet. It has an earth connection and built-in fuse:

Ooh - another one with no CE, UL, CSA etc marks.

I'm thinking of fitting it to the inside of a cheap combination key safe, such as in eBay item #361193885359 (£12) and using intumescent sealant where cables enter and exit. That should do the trick, shouldn't it?

It will certainly help it to overheat.

Do you know if this PSU is a switched-mode PSU?

Given its size and wide range of input voltages it has to be.

 

[bernardgreen]

Looking at that debris it would appear that the PCB was assembled by hand. to a poor standard. Component leads bent by hand and not pre-formed is a clear sign of cheap labour without the correct equipment. Could have been assembled on the kitchen table ( or floor ) by an outworker's children.

Intitial failure could be due to the poor assembly ( bad solder joint ? ) but the resultant fire was due to bad design and / or cheap components. The resistor that vapourised was probably in series with the mains feeding the diodes forming the bridge rectifier. If that resistor had been a fusible resistor it would have gone open circuit in a safe way and thus disconnected the mains without it becoming a miniature carbon arc lamp burnimg until it finally disintegrated.

 

[ban-all-sheds]

Chinese imports are a problem where they do not conform with safety standards however they are cheap.

Avoid cheap ( and therefor often nasty ) equipment.

Prevention is better than cure - as Bernard says, start by stopping buying cheap and flaky equipment.

your first step should be to replace all of your dodgy PSUs with reputable ones.

 

[endecotp]

Thanks for the photos - very interesting.

The case looks rather narrow for a UK plugtop, but maybe that's an illusion.

I can think of a couple of possibilities for what the failed component was; it could be a bleeder resistor to discharge the capacitor when the unit is unplugged, or it could be to feed a bootstrap voltage to the control circuit. In the latter case you would typically have two or three resistors in series to avoid any one of them failing and the DC output becoming live. It's hard to know more without examining it.

I really don't think that earthing is relevant here.

I don't think you need to go to the extremes that you have suggested above e.g. metal boxes (and as BAS pointed out that could result in overheating). Just buy some reputable plugtops. There are companies like sunpower who should be able to sell you something suitable, especially if you want several:

http://www.sunpower-uk.com/ranges/Plugtop-Power-Supply/42/default.htm
http://www.sunpower-uk.com/public/ranges/pdfs/r400/r400_1.pdf