Electricity Underwater- whats safe?

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Keiron

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This is a slightly strange marine related question and any advice is gratefully received.

Is a current of 1.1amps safe underwater for someone diving/swimming near it or touching it? Its produced from a 12v source and is from a DC transformer running off the mains? The equipment the electricity is powering is fully waterproofed, but where it is being used it has a high change of being damaged. Hypothetically 70mA can kill you hence RCDs are set at 30mA. Should this power supply be protected by an RCD ? Why is AC more dangerous than DC?
 
When the body is affected by an electrical charge, muscles contract.

DC shock makes you grab and hold on to the source.
AC shock makes you grab and then let go - hence the "fluttering " feeling when you stick fingers in light sockets.

I'd be rather worried authorising anyone to work with the kind of setup you are describing, without knowing a lot more about the design side. Is the transformer above water, or below ? What kind of fuses are in the circuits ? Slow bow / quick blow ? Is the circuitry earthed ? How ? Why is the equipment likely to become damaged ? What mitigation can you undertake to reduce the chance of damage or exposure ?


How kind is your insurance company ? /forums/images/graemlins/grin.gif
 
I assume you mean that the equipment attached daws 1.1A? The current flowing in the load is not relevant to how dangerous it is - it it the current flowing through YOU that will kill you! I do not believe that 12V wiring presents any realistic shock hazard.

Having said that, 12V is usually considered to be pretty safe, either in the water or out. I'm not sure that AC will be any different to DC as far as safety is concerned. I guess when most people say AC they are thinking of the "mains" supply, which is 230V - altogether more dangerous.

If you are worried about cable damage, then maybe you should consider armoured cable.

You don't say if it's fresh water or salt. Sea water conducts electricity fairly well, while fresh doesn't. In either case, I still think you are safe at 12V.

More important is to ensure the power supply is safe - that SHOULD be protected by an RCD. If a fault put 240V on your cable, that would be deadly!
 
Thanks to you both for your responses. Sorry to clarify the transformer is above water and in fact a maximum voltage of 30v could be delivered underwater. The fuses are fast blow fuses. The circuit is full earthed on the surface via a fairly standard UK style electrical system. The cables are likely to get damaged by floating ice although we will try to protect them as best as possible. The divers will be wearing suits and gloves but ideally the equipment will be live when they check it.

Yep the equipment draws a maximum of 1.1amps. Its in seawater. I presume if the power supply failed and delivered 240v through the cable the RCD should cut out?

Can't tell you what the job is or I will have to kill you! Needless to say its a commercial diving job and I'd like to ensure the info I have been provided with is correct, as I will probably be in the water doing the job! Thanks again.
 
The difference between AC shock and DC shock is that with AC your heart tries to beat at 50cycles a second which it obviously cant do, so you die if not pulled off quickly.The system you are talking about ,I would not use as the transformer if not designed properly could get an internal short which could increase the the voltage to dangerous levels,You could get a transformer where the high and low voltage windings are separated from one another, but why not use a 12volt battery which would be 100% safe from shock.
 
I am still unhappy about the design environment.

The device (drill, saw, or whatever) will presumably only be in use for - say - an hour, given that divers have a limited time below.

So, as suggested, why not a local 12v supply, or even a compressed air solution ?
 
OK good advice on the transformer. Unfortunately 12v batteries are not really an option. The equipment will be left in situ and powered for 12months or more. Basically the equipment is composed of a small (~20cm x 20cm) plate which will be heated. There are multiple such units. The correct amount of heating is controlled by the the voltage/watts and a range of temperatures are required hence voltages between 12 and 30v. If a transformer did short internally would an RCD trip? I would guess so. Is this a common problem with transformers?
 
OK, getting the picture.

You need some very definite MTBF (mean time between failure) figures from the transformer manufacturer (basically guaranteeing that the unit will work for 5000 hours, or what ever you want).

If a short occurs in the transformer, it depends on where the current moves from its intended path. A properly specced RCD will act very quickly for an earth related fault, but a fuse is probably better for an overload type failure.

A guess: are these to prevent small switches/valves from freezing ? Maybe people like thses could help from their existing range ?

http://www.conrad-co.com/
 
There are transformers and transformers....some have a common winding with tap off points to give the 240 /12 volts which means that in the event of a winding failure there is a risk of high voltage in the secondary circuit (autotransformer)

The more suitable transformer as a couple of posters have suggested is the isolating transformer with totally separate primary and secondary windings this type would give added protection however that is all. I believe in this James Bond world you might never resurface if the contents of a AAA battery are discharged to the right part of your body!!!!!!!! and that is only 1.5 volts.

The divers wont be touching these items will they? maybe just swimming in the vicinity,
 
Erm...

Bilgediver refers to "autotransformers" but these are impossible to come by, unless custom made (at least for 12V output), so it is not likely to be one of those!

Most transformers these days are designed for enhanced safety, in the absence of a protective earth. In general these will be "double insulated" and identified with a double concentric squares mark printed on them. These require TWO safety elements (insulation layers) to have failed before a safety hazzard exists. If you combine this with a RCD on the primary (mains) side, you will be as safe as can be provided. Needless to say, the physical design of the power supply must follow the same principles - no single fault can cause a shock hazzard. There is a BS/EN standard for this kind of thing, I forget which it is but any medical or telecom testing company will be able to advise. It is important to ensure that if a wire falls off, or insulation gets worn off there is still another layer of protection.

If the transformer insulation failed internally, so that the output became live, as soon as there was a failure in the insulation of the underwater part also then a leakage current would flow which would trip the RCD - long before one of your divers were exposed to it. The insulation failure in the underwater part would not have to be much for current to flow - just a crack, or an imperfectly sealed joint would be enough.
 
Re: Electricity Underwater- whats safe?

What about a 12 vot battery charged by a mains charger - you then have total isolation.

However, having read about low current shock, there is potential for a diver to be panicked, and have an accident - embolism or otherwise........
 
Re: Electricity Underwater- whats safe?

It should be relatively easy to isolate and make safe the 240V side from the 12 volt. If you had any real doubts you could use yet another transformer to isolate the 240V supply from the 240V winding with an isolation transformer or a 12v to 12v transformer.

The 12v supply itself can be a danger considering that if you wear a wet suit then you have huge conductive salt water connections to the area around the heart. 12v could conceivably be deadly. However of course you have the conductivity of the water to bypass any current leakage.

In some fields a Faraday shield is used to protect people from induced voltage. This consists of a conductive metal suit which provides a low resistance path from all points of the body to all other points.
I somehow doubt however that you have much risk of electrocution.
Especially as I am guessing you will wear a dry suit.
You might consider wrapping your body in foil to bypass any current near your heart. Or even connect foil in each glove to foil in the other glove by a wire to avert hand to hand current flow.

To Sarabande I was taught that AC will cause the muscles of eg the hand to grip onto a conductor while DC will tend to spasm the muscles to release the conductor.

If it is a commercial operation then surely there must be standards of safety for such operations. It behoves you to find and comply with the standards. Even if there is little risk you must be seen to do all possible to minimise risk.
good luck olewill
 
Re: Electricity Underwater- whats safe?

[ QUOTE ]
Sarabande I was taught that AC will cause the muscles of eg the hand to grip onto a conductor while DC will tend to spasm the muscles to release the conductor

[/ QUOTE ]

I understand from reading stuff on the southern railways where they use very high voltage DC currents on the third rail- that you hold on to the conductor - and you literally fry - heads have been known to blow off!
 
Re: Electricity Underwater- whats safe?

After we have concidered people, what about the boat, is 12 volt DC a good thing in the water ?

Brian
 
Re: Electricity Underwater- whats safe?

If I was working underwater with 12v DC it wouldnt worry me,but if I was dependent on an RCD it would worry me. I look after our boat club mooring power points which are protected by RCD,s and MCB,s. We have approx 68 sockets, I check them every 6mths and I usually have to change about 3 or 4 RCD,s for failing to trip on earth checks.So anyone out there dont forget to try your test button on your RCD.
 
Re: Electricity Underwater- whats safe?

[ QUOTE ]
After we have concidered people, what about the boat, is 12 volt DC a good thing in the water ?

Brian

[/ QUOTE ]

Brian - I wondered about that. I certainly would not want something like that beneath my boat!

Is this some kind of ice clearance thingy? I thought they used comperessed air bubbles for that.

My son designs deap sea seals for electrics in the oil industry - a seal is apparently very difficult to achieve over a period of time - but then they are rather deaper... /forums/images/graemlins/smirk.gif
 
Re: Electricity Underwater- whats safe?

Not worried about the person and 12 volt, it was the boat, if you have 12 volt floating around the hull and all the fittings.

Brian
 
Re: Electricity Underwater- whats safe?

Many thanks for all the advice its been a big help. Don't worry the project isn't going to be anywhere near any of your boats!
 
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