What kind of safeguard would protect against mains over voltage event?

[IanB]

Thinking in particular about this story:

http://www.bbc.co.uk/news/uk-england-lancashire-15867961

A power company fault introduced extreme over voltage into many homes and destroyed everything electrical that was plugged in.

Every now and then a fault like this is reported, due to crossed wires, short circuits, fallen trees or some such. It seems like some protective device where the mains enters the house could detect such a problem and isolate the supply in milliseconds before damage occurs. Is there a standard item that might do this at reasonable cost? Could some kind of controlled breakdown device be used to trigger a standard ground fault/earth leakage detector?

[amspire]

They don't detail if the initial cause was a high voltage DC surge or a sustained high voltage AC.

If it is a DC surge, there is absolutely no cheap solution. A 5KV DC breaker is a very large and very expensive device, and by the time the arc was quenched in the breaker, all the electronic stuff is dead.

Protecting against AC is a little easier, but to design some kind of black box that eliminates  HV totally from a house is again extremely expensive. Definitely starting at thousands of dollars at least. In most circumstances it is cheaper just replacing equipment that does get fried, so the only people who are prepared to pay for full protection are the ones who would lose thousands a day if their equipment died.

Back in the late 80's when I was designing UPS equipment, we did design gear that would probably fully protect against this kind of fault, but you had to explain to customers why they needed to pay $3000 to protect their $2000 PC against something that has perhaps a one in fifty chance of occurring. Not many people are prepared to pay.

Richard

[IanB]

Reading between the lines of the story it seems like a sustained high voltage was applied, since items are described as smoking, melting and exploding. Maybe somehow the 415 V line voltage got supplied instead of the normal 240 V phase voltage. But in that case why wouldn't fuses and breakers just trip with the high fault currents?

Or maybe the 11 kV distribution voltage crossed over onto the low voltage side. In that case I suppose there is not much to be done. But if it was 11 kV then I imagine there would be more damage than just killed domestic appliances. It would cause arcing, fires, and all sorts of mayhem.

[amspire]

The only real way you can protect electronics from very high applied voltages is to totally isolate the household power from the mains through an inverter, ferroresonant transformer or a motor/generator pair.

After you have installed all this, your household electronics is safe, but the $10,000 thing you have converting the street mains to the household power is still vulnerable to be fried, so you end up back where you started.

Semiconductors can die in microseconds, and most HV protection is slow - even seconds. Fuses are way to slow particularly for DC surges.

If someone did apply 11KV to the street, then everything plugged in has absolutely no chance - even if the mains power switch is off. There is no practical protection you could add to save anything.

Perhaps someone managed to wire a 11KV transformer back to front. That would be fun!

Richard

[AntiProtonBoy]

You guys are scaring me. Stop it.

[NiHaoMike]

They make surge protectors designed to withstand near (indirect) lightning strikes. I presume there are even some designed to withstand direct lightning strikes and nuclear EMP, although they're likely to be unaffordable for the home user.

[amspire]

They make surge protectors designed to withstand near (indirect) lightning strikes. I presume there are even some designed to withstand direct lightning strikes and nuclear EMP, although they're likely to be unaffordable for the home user.

There are only two ways to protect against a surge if you are directly connected to the street mains - either absorb the power, or disconnect from the power.

With HV, disconnecting is extremely difficult as it loves to arc.

Devices like MOV's can absorb a lot of power for an extremely short time, and after that, they just catch fire.  Even when MOVs absorb power, a big surge will still put 800V + across mains powered devices so they can still blow - they just probably will not have the big arcing paths all over the board that you see usually see with a lightening strike.

As I mentioned, whenever you want complete protection, the only solution is isolation  All the best UPSes isolate the loads from the mains. So you want nothing in the house to have any direct connection to the power from the street.

Every way that attempts to protect any device that has a direct mains connection is only successful at the low end of energy faults.

It is not that it is unaffordable, it is just that people decide it is uneconomical.  It would cost something like $10,000 + per house for a proper system to protect against something that is unlikely to happen. Most people just want to spend that $10,000 on something else and risk it.

Richard

[IanB]

I realize a few thousand volts is going to be a bit of a problem, but I'm more curious about a few hundred volts, which is perhaps a more typical power company fault. Shouldn't a normal circuit breaker be able to isolate that when it trips? I'm thinking in terms of something that trips within one or two cycles when a peak voltage somewhat above the maximum is detected. This device could be put downstream of something that absorbs short spikes such as arising from a lightning strike nearby, to guard against false alarms. In particular, suppose you put a large ELCB/GFCI on the mains inlet, and then put some device with a breakdown voltage around 500 V between live and earth. As soon as the voltage was too high this would  trip the ELCB and isolate the house.

[NiHaoMike]

If the problem is a really short spike (lightning, for example), wouldn't series inductance and shunt capacitance attenuate it? Space out the inductor enough and the high voltage will arc to ground elsewhere. Legend says that merely tying knots in the cable works for lightning protection, although I don't see how it would work.

As for more long term overvoltage, what about a fuse packed with explosive to blow out the arc and throw the wire right out of it? Such a device can obviously only be used outdoors, but I can see it having a really good interrupting voltage rating.

[david77]

I've put a couple of these guys in to our distribution panel

http://www.dehn.de/cgi-bin/pdbWWWEF/extpublicpdb.cgi?ikatpreviewue:100020::::rl:::::::int

protecting the office (server, networking stuff, computers, PBX) and the living room (TV, stereo) from
power surges.
The rest of the flat is unprotected or protected by the plug-in type protectors, these Dehn surge protectors are
rather expensive.

I expect them to work in case of (minor) surges or if the power company should make a mess of their wiring and put
400V through to consumer units on the wrong wires, it does happen more often than one would think.

[Rufus]

Maybe somehow the 415 V line voltage got supplied instead of the normal 240 V phase voltage. But in that case why wouldn't fuses and breakers just trip with the high fault currents?

The most likely fault would be loss of neutral leaving you with up to 415v depending on load balance between phases. Maybe they lost a phase as well ensuring a big imbalance.

Happened here once. I noticed unusual transformer humming and measured the (nominal 230v) mains at 270v. Turning things off makes it worse, I turned on a 3kW heater and dropped my phase voltage by about 20v. The power was cut and they were fixing the cable within a couple of hours.

I think it was an underground 3ph cable with aluminium sheath carrying neutral, if the overall sheath gets damaged the aluminium corrodes eventually leaving you without a neutral.

[saturation]

A surge arrestor used on the mains feed to the distribution panel then plugin units to protect item plugged into branch circuit receptacles is a simple comprehensive way.  You can download a NIST brochure on the basics of this process.  Although it covers mostly surges, I recall it discusses issues with power line quality too.

http://www.nist.gov/public_affairs/practiceguides/surgesfnl.pdf

MOVs work by shunting excess energy to ground, so even if your neutral is disconnected or other anomalies occur on the hot line, the MOVs will conduct all excess above its let through voltage to ground.  So from a home or facility perspective, the final crucial protection relies on truly electrical code quality ground.

If MOVs are unable to shunt energy to ground fast enough they will blow.  The panel arrestors are made in housings to withstand fire and flare, at least to UL and the US electrical code specifications and tested for same.  This would then disconnect all power to the facility.  You'll need an electrician visit but it should save most if not all your electrically powered devices and appliances.  I think this method is typically used in many mission critical areas like airports, hospitals and military bases.

Any spikes created within the facility wiring that enters the wiring by EMF after the arrestor or through non-power line transmission should be covered by the plugin branch circuit surge protectors.  They may not all succeed but it should spare most devices if the surge protector was made properly.

[Simon]

what about some sort of crowbar that will blow a long fuse preventing arcing ?

[Mint.]

I've got a short circuit system installed in my house. Whenever something short circuits or overloads, everything in my house gets automatically switched off. All I have to do is go to the electrical box, turn the switch back on and it's all good. Sometimes it has a few false alarms, but overall it's a handy option.

[vk6zgo]

Statistics & prayer,in that order.

VK6ZGO

[PetrosA]

There are a number of potential causes for this type of incident. Here in the US, where primary lines are run above ground, it often happens when a vehicle hits a pole that carries both 13.2kV lines above 4.1kV lines and the 13.2kV lines drop onto the 4.1kV lines, which will shoot triple the normal voltage through homes and businesses for as long as it takes to burn out the small transformers feeding individual customers. When a 120/240 system gets a jolt of 360/720 or so, not much will survive and there's not much you can do to save anything that's turned on. If, in addition to the higher voltage, the transformer burns out in such a way that it fuses the primary side to the secondary side and you get 13.2kV directly into the home, not even the devices which aren't on will survive. I've seen light bulbs explode from this type of hit (scared the shit out of me...).

For the more common surges and spikes not related to massive faults or direct lightning strikes, lots of good quality, plug-in surge protection is your best bet. The more you have distributed around the house, the more voltage can be absorbed. I'm also a big fan of using decent quality line-interactive UPS's. I'd much rather lose a $300 UPS than all the files on my hard disks.

[NiHaoMike]

Wouldn't a crowbar work for long term (more than a few ms) overvoltage? Surplus SCRs with really high current ratings are surprisingly cheap. Add proper ground bonding to make fault currents flow around equipment and they should survive or suffer less damage.

Perhaps a relatively cheap crowbar could consist of MOVs or other surge protection devices contained in a sealed metal box along with highly conductive solder or low melting point metal. If really overloaded, the solder would melt and permanently short it.

[Simon]

I was talking to a guy last night about this discussion and he said that the most common issue was loss of a neutral that causes 415 volts to go around. It happened to him not long ago but luckily he lost power rather than have a surge.

suppose so sort of crowbar from live to earth activates the RCD ? surely an RCD will isolate up to 500 V ?

[jahonen]

suppose so sort of crowbar from live to earth activates the RCD ? surely an RCD will isolate up to 500 V ?

RCD would most certainly work and it could be tripped with just a VDR between live and earth if such event would occur. But I wonder if that is fast enough to prevent serious damage.

Regards,
Janne

[Simon]

well the article said that one woman reported that the spot lights went "one by one" so the actual event took some time and was not a massive spike but more like a rise in the supply voltage so in comparison the trip time of an RCd is already a great step in the right direction, maybe set the crowbar to earth at as low a voltage as possible and have a bank of movs or TVS's at a little more than that voltage to help stop thing going to high while the RCD trips.

[saturation]

It certainly can.  There are many methods.

I think MOV based designs have risen to be a common method as its very effective, low cost and easy to apply; a electrically wise owner could install one of the whole home surge devices after the distribution panel and save some money.  However power surge could destroy the panel too and thus even if appliances are spared, the owner would have to have a new panel installed.  Which is better, put the protector before or after the panel I don't know.  If the MOVs trip, the after panel version is user serviceable, if the it trips before the panel, it may not be. 

I would definitely install such a device in a home.  In buildings I live or work in, similar devices are installed for the main circuits providing power

http://en.wikipedia.org/wiki/Surge_protector

what about some sort of crowbar that will blow a long fuse preventing arcing ?

[electr_peter]

Looking at this old story it seems that there is a cheapish way to protect from such faults.

Fault was a slight overvoltage event. If house entrance panel had a voltage monitor which could detect overvoltage (300V instead of 220V), neutral and live could be disconnected via relays, preventing disaster down the line.
Slight overvoltage is not an issue with main panel/fuses/relays, unlike household items.

[Simon]

Looking at this old story it seems that there is a cheapish way to protect from such faults.

Fault was a slight overvoltage event. If house entrance panel had a voltage monitor which could detect overvoltage (300V instead of 220V), neutral and live could be disconnected via relays, preventing disaster down the line.
Slight overvoltage is not an issue with main panel/fuses/relays, unlike household items.

You have dragged up a 3 year old topic to state what was stated just above....