Author Topic: Electrical safety in caves  (Read 1379 times)

Offline Chocolate fireguard

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Electrical safety in caves
« on: January 09, 2017, 12:56:23 pm »
There is an active topic in the Peak District section where the subject of running an electrical supply to a distant underground pump is briefly discussed.
I think similar things have been done by some in the caving community in the past on an ad hoc basis but I have never seen anything you could call a technical discussion on the subject, by which I mean an attempt to answer some (fairly obvious?) questions such as:

What voltage is considered safe in a caving environment (and with reasons)?

Does the supply need to be earthed?

If so, and you are using a portable generator out in the sticks, how do you go about doing that?

Is an RCD needed as well as the fuse/mcb?

Is putting up with cable resistance dropping the voltage, then using a transformer at the end to raise it again, acceptable - bearing in mind that if the pump/whatever is not running the full voltage appears in the cave?

Is it advisable to use one of the modern systems that provide a supply isolated from earth but disconnect when a break in each conductor is detected?


I am not looking for a discussion on voltage drop/cable size.

If I was in a low wet cave passage alongside a cable carrying a 230V supply, let alone sharing the Mucky Ducks or the Wet Wallows with a 2kV supply, I would want to know that the people who had installed it had given a bit of thought to the above questions.



Offline crickleymal

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Re: Electrical safety in caves
« Reply #1 on: January 09, 2017, 01:15:57 pm »
One place I worked at had a safe bench. The mains supply to the bench was isolated using a transformer so that if you then touched the live wire on the bench it couldn't give you a shock as it had no reference to the ground. So if you're using a genny then you should be ok provided the genny has no connection to the ground it's sitting on. If it has then fit an isolating transformer.

As for voltage drop down a cable. Hmmm... If the resistance of the cable is such that it is dropping so much voltage that you need a step up transformer then the cable will probably melt. A step up stranformer won't replace the lost power which is being dissipated as heat in the cable (due to the resistance of the cable). Most step up trannys will probably be 120-240 so if you've lost 120V you'd be in deep doo doo.
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Offline SamT

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Re: Electrical safety in caves
« Reply #2 on: January 09, 2017, 01:44:45 pm »

Calling Nick, UKc Calling Nick are you reading.

Anecdotally - we had a 110v genny on the surface, with 50m odd of cable running down to the digface to run a drill, It was fairly chunky cable IIRC.
The dig was quite a damp spot with lots of splashy drippers. eventually the drill got damper and damper until it was getting quite 'jumpy' to hold.  There was a comedy moment when we kept passing the drill back and fourth to each other, like a grenade with the pin pulled.  We decided to make for surface at that point. 
Nobody died.

Waterways swallet diggers had a cable running to the bottom of the cave which must be at least 150 meters.


Offline andrewmc

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Re: Electrical safety in caves
« Reply #3 on: January 09, 2017, 01:48:17 pm »
Depends how long the cable is. If you are losing half your voltage in the cable then you are dissipating the same power in the cable the load. If the load is say 500W then 500W in 1km is not much heating at all. 1kW at 250V is only 4A which is peanuts.

I hadn't heard of a Variac before. It sounds like a great solution.

Offline TheBitterEnd

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Re: Electrical safety in caves
« Reply #4 on: January 09, 2017, 07:17:28 pm »
FWIW the supplies used on construction sites (yellow transformers, yellow plugs/sockets etc.) are 110V and centre tap earthed, i.e. 55V above earth and 55V below earth - you would still feel it but less likely to be fatal. But lower voltage means thicker cable to reduce losses.
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Offline PeteHall

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Re: Electrical safety in caves
« Reply #5 on: January 09, 2017, 09:15:05 pm »
We used to run pumps, lights, ventilation and a cement mixer off a 240V feed from a genny on the surface. Probably about 100m of cable total.

We kept everything on RCD's and used IP66 components. Wouldn't pass a risk assesment for a workplace, but never had any problems (either near misses or tripping out).

I have also heard of people using small generators close to point of use, but obviously needs good ventilation!  :o
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Offline nickwilliams

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Re: Electrical safety in caves
« Reply #6 on: January 09, 2017, 11:09:45 pm »
Some brief points 'cos it's late:

- Yes, use an RCD.

- You can use any voltage you like so long as it's properly insulated and waterproof. Don't go above 50V if it's not.

- Use an RCD. At the generator end.

- If the equipment you are powering is class 1 (earthed) then your generator should be as well, if all your gear is class II then the generator earth isn't going to do a lot.

- Use an RCD. At the generator end. Ideally a 10mA trip current one.

- Any generator which is switchable between 230 and 110V output won't have a proper earth connection. Lots of bigger ones don't either.

- Use an RCD. At the generator end. A 10mA one. And test it regularly.

Do you agree, Chocolate Fireguard? 'cos I know you're trolling in a fairly gentlemanly fashion, aren't you!
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Offline andrewmc

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Re: Electrical safety in caves
« Reply #7 on: January 10, 2017, 12:49:59 am »
I have also heard of people using small generators close to point of use, but obviously needs good ventilation!  :o

At what point is piping the exhaust out easier than getting the electricity in, I guess?

Offline caver.adam

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Re: Electrical safety in caves
« Reply #8 on: January 10, 2017, 03:45:04 pm »
If I'm reading this right you're only asking about safety. So here's my 2 cents - take them as unqualified advice.

Water and electricity don't mix. Keep the wires as dry as possible. Especially the connectors. But, water inside the insulating covers can cause corrosion of the wire so water will hurt the wires if you keep them in the cave long term.

Do your wiring with the power off. As in totally disconnected from the power source (generator off or wires unplugged from generator).

If you are worried, you can put an amp meter on your power output. If you have current output on your wires with no appliances hooked up then you have a short somewhere that you need to fix. Turn off and unplug wires before looking for the problem.

Grounding is required for some equipment. Metal bodied drills often require grounding because it is possible for a charge to build on the metal body and discharge through a person into ground.  You can set up a grounding wire at your project site the same way that you do for a house. Look up how to install a "grounding rod". You should be able to get one at your local home improvement store. And installation just requires driving deep enough into your soil (depths may vary by location).

Offline nickwilliams

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Re: Electrical safety in caves
« Reply #9 on: January 10, 2017, 06:21:55 pm »
This is where we start to get into the detail (which I guess was what Choc. Fireguard really wanted). There's several hours work in addressing all these questions and I don't have time for it today, but I can address the one about earthing to some extent.

(For those who worry about these things, my background is that I have a degree in physics and have spent 30+ years in electrical and electronic equipment design and manufacturing, the last 25 of which I have spent running a product safety consultancy specialising in electrical and machinery safety. I'm not a qualified electrician, but I'm a Technical Director for a Notified Body and one of their appointed technical experts for EMC, Machinery and (until the LVD was revised in April last year) electrical equipment.)

Grounding ('earthing' in the UK - I guess caver.adam is one of our transatlantic bretheren?) is not required to deal with a build up of charge on the metal body of equipment. Grounding is required because some items of equipment (known as ‘Class I equipment’) only have a single layer of insulation between live parts ('live' = parts at dangerous voltages) and external metal parts so a single fault could result in those external metal parts becoming live and causing electrocution. To prevent this, the external metal parts must be reliably connected to an earthed supply conductor. One side of the supply is also connected to the earth, so when there is a fault, current flows. The trick is to ensure that the resistance of the ground circuit is low enough that a substantial current flows since this then causes the fuse (or circuit breaker) at the supply end to blow and cuts off the electricity supply to the faulty equipment. So long as this happens quickly enough (i.e. within a few tens of milliseconds) then the chances of electrocution are limited even if someone is in contact with the live metal.

The connection to earth at the supply end is done by connecting one side of the generator windings to ground. For a small generator this can be done by connecting the neutral side of the windings to an earth rod. The main problem with this is that some small generators don't have an earth terminal you can connect a ground rod to. The other big problem is that a single earth rod may not give a low enough resistance to allow enough current to flow in the event of a fault for the circuit to be broken quickly by the fuse. Unless you check this 'earth loop' resistance every time you install the generator, you cannot be sure earthing protection will work. In practice, most small generators are simply plonked on a convenient piece of ground and no earth rod is ever installed. In these circumstances it’s anybody’s guess what the earth loop resistance will be, but it’s very unlikely to be low enough to allow the fuse to blow in the event of a fault.

A further problem with small generators is that they may not actually have a connection between the chassis and the windings, and especially for those sold in the UK, if they are switchable between 230V and 110V output then it’s physically impossible for the earth connection to be in the right place on the windings for both outputs to be properly earthed.

There is an argument which says that if the generator circuit is completely isolated from ground then there is no path back to earth so even if the accessible metal parts of the equipment being powered become live there is no electrocution risk because no current can flow. This is what’s known as ‘protection by separation’. In practice, the current required for electrocution is so low (a few tens of milliamps) in comparison to the current the generator is actually capable of supplying (a few tens of amps) that unless a great deal of care is taken to ensure the insulation can always be relied on, you can’t be sure the circuit is safe. Generally speaking, earthing things is a more reliable way of being sure you know what’s going on, and actually a lot of the time ‘earthing’ will work even if there is is no connection via an earth rod because the external metal parts of the equipment and the chassis of the generator are connected together by an earth wire in the supply cable which acts as an artificial earth. However, for that to work, the generator windings actually have to be connected to the earth terminal on the generator socket, and as has already been said, that isn’t always the case.

Class II (double insulated) appliances and the use of residual current breakers (RCD’s) both have big part to play in compensating for the deficiencies of small generators when powering class I equipment, but that discussion will have to wait for another day.

Others may care to chip in in the meanwhile.
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Offline Chocolate fireguard

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Re: Electrical safety in caves
« Reply #10 on: January 10, 2017, 07:53:26 pm »
Some brief points 'cos it's late:

- Yes, use an RCD.

- You can use any voltage you like so long as it's properly insulated and waterproof. Don't go above 50V if it's not.

- Use an RCD. At the generator end.

- If the equipment you are powering is class 1 (earthed) then your generator should be as well, if all your gear is class II then the generator earth isn't going to do a lot.

- Use an RCD. At the generator end. Ideally a 10mA trip current one.

- Any generator which is switchable between 230 and 110V output won't have a proper earth connection. Lots of bigger ones don't either.

- Use an RCD. At the generator end. A 10mA one. And test it regularly.

Do you agree, Chocolate Fireguard? 'cos I know you're trolling in a fairly gentlemanly fashion, aren't you!

Thanks for the gentlemanly bit but no I am not trolling.

I assume the show caves and such like have the safety issue cracked, but your average caving club won't have the time, money or space to wrestle with armoured cable and pin it safely out of the way of the punters so when I read the bit in the Ink Sump thread I wondered  if anybody really knew how (or perhaps if) they could safely go about running a decent-size pump several hundred metres into a wet cave, quite possibly using a generator and having to provide their own connection to earth. Although on an Ink Sump job they would at least be spared that last.
I have thought about it on & off for a few years - for no reason other than it interests me.

Nick has well described the problem providing and checking a decent earth connection for the generator.
If you want a fault to blow a 10A fuse within a few seconds the loop impedance needs to be less than 10 ohms. That includes the resistance of the live wire.
I doubt that is possible in shallow soil without some major earthmoving to bury a metal grid.   

It seems to me that it's not the equipment at the end that will be the main danger - despite Sam's fun with the drill - it's the cable that people will have to crawl beside/over.
With the best will in the world, over the lifetime of the project it might not always get the care it deserves, it will probably have joins in it and it may not have been in the first flush of youth when it was installed. A break in the insulation of the live (if its an earth, live & neutral system) may not cause enough fault current to blow the fuse, but will drive the fatal 10s of mA through a person lying in the mud beside it. With a system protected by separation breaks in the insulation of both wires would be needed. An RCD might offer protection in the first case, but not the second.

However, the thing I really wanted comments on (but nobody picked up on it) was the system I referred to in my 6th question. I have tried unsuccessfully to find out more about it and whether it could be applied in caves, but I have got nothing back from the Technical departments of the compenies who market it.

Briefly. its a 2 wire system (protection by separation, so not earthed) but where the resistance to earth of each conductor is monitored. I know that sounds like nonsense, but in practice no insulation is perfect, and capacitive effects will produce small currents, so even when there is no direct connection to earth there will not be perfect insulation. Typically in a hospital theatre (where these are fitted, on the Continent) that resistance will be many hundreds of Megohms. In a cave it will be very much lower.
If an earth fault starts to develop in one of the live wires this resistance falls. An alarm sounds but the supply is not disconnected - after all, there is no dangerouse situation, the system has effectively become live, neutral and earth.
Obviously if the second wire starts to go down the supply is disconnected by the monitoring electronics.

Any information from anyone who has experience of this system would be gratefully received.





Offline PeteHall

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Re: Electrical safety in caves
« Reply #11 on: January 11, 2017, 09:18:05 pm »
I can't comment on your 2 wire system as I have no evperience of it, though it does sound interesting, but one thing to add to Nick's list, that I didn't mention first time around is an LED indicator close to point of use.

There could be any number of reasons why things don't work at the end of your dig, but a little LED indicator will let you know if you need to go out to re-fuel/ re-start the genny, or if you are looking for a fault at the equipment end.
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Offline pete_the_caver

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Re: Electrical safety in caves
« Reply #12 on: March 09, 2017, 09:40:49 am »
Many years ago, some Swiss friends were doing a lot of drilling deep in one of their caves and faced the same issue. They placed a generator on the surface producing 220V this was bumped up to 440V for transfer to the dig site upon where the voltage was reduced to 24V. If you're worried about issues with cable safety you could use armoured through areas where damage is possible and make sure you have an RCD


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