What is an acceptable level of earth leakage?

[Gerry]

Greetings

A boat near mine is tripping the earth leakage detectors on the pontoon. The owner says that the current is only one mA and therefore no problem. PBO's book Electrics Afloat says 2OOmA at 60V is enough to kill.

This is of some concern because I do dive on my boat to change anodes, clean the hull and so on.

Does anyone know what is a lethal current at 110V and at 240V? What level of current, if not potentially lethal, is sufficient to create corrosion problems in nearby boats?

What is the legal requirement? I know some marinas won't let you plug in if they detect any leakage at all. Once you are registering some leakage, is 1 mA to 200mA such a big step?

I would welcome any thoughts on this.

Gerry

 

[Solent sailer]

Hello
Anything over 15mA may kill, the voltage doesn’t matter it is only the "pressure" pushing the current and the current is what will kill you!
The time is another critical factor which is why most domestic RCD's (residual current devices) as used on your lawn mower will normally trip within around 20mS. The BS standard or BSEN requires that this type of RCD operated within 40mS at 150mA.
I would not expect any equipment on a boat to have any leakage, much of the electrical equipment at home will leak to earth typically electrical hot plates and computer equipment will have an earth leakage, the hot plates through degradation in the elements exacerbated by damp and not being used, computer equipment and some other electronic equipment like microwave ovens by the filter networks used in there power supplies.
The minimum insulation resistance for an installation is 1Mohm this would give a leakage current of I = V / R 230/1000000=0.23mA.
However it is permissible to have circuits that contain equipment with a high earth leakage and it is not necessary to carry out any special safety precaution unless the design leakage for the circuit is greater than 10mA.
To sum up your friend is correct 1mA does not contravene any regulations (in my view) providing it is connected equipment causing the leakage and not the fixed wiring, if the leakage is caused by the electrical installation itself he may have a problem.
Most electrical equipment used on a boat will be "Class 2" no earth used and protected by a second layer of insulation (battery chargers, fan heaters etc) which is why I am surprised the leakage current is so high. I am also curious how it was measured as obtaining an accurate earth leakage measurement with all the parallel paths available for earth leakage is not easy and I would be rather suspicious of any DIY readings quoted.

 

[oldestgit]

Hi Paul:
You seem to know what you are talking about, but what if any use would a Galvanic Isolator be in this case and do they actually save the anodes.

Regards... Peter

 

[john_morris_uk]

I know I am not Paul, but the answer is sort of embedded in Paul's original reply.

Because of the multiple possible paths to earth, (including via your skin fittings and the surrounding sea-water) the Galvanic Isolator prevents small stray currents by requiring about 0.7 Volts to overcome the forward breakdown voltage of diodes. The diodes are wired in such a way that they don't (cannot) rectify the AC voltage, but do allow higher currents to pass and maintain the safety features of the installation and normal ground connection.

The reason that they are so expensive is that they need to be proof against high overload currents. Diodes are very fats fuses if you are not careful.

 

[Norman_E]

I bow to Paul's obvious expertise, but am very skeptical about the boat owner's claim that the current is only 1 mA.
I share Paul's doubts about the accuracy of this simply because most RCD units are rated to trip at higher current levels than that. It is possible that 1 mA will trip them, but I have my doubts wheter all are that sensitive. I think you need to complain to the Marina operator, especially if the tripped RCD is cutting the current to your boat as well as the offender.

 

[VicS]

[ QUOTE ]
Galvanic Isolator prevents small stray currents by requiring about 0.7 Volts to overcome the forward breakdown voltage of diodes. The diodes are wired in such a way that they don't (cannot) rectify the AC voltage, but do allow higher currents to pass

[/ QUOTE ]

Thats almost right but not quite. Galvanic isolators block current from very low voltage sources (actually up to about double the figure you quote) but allow current from higher voltages sources (Eg mains electricity) to pass so that the effectivness of the earthing sytem and is maintained.

 

[2Tizwoz]

'In May 2002, a 14-year-old girl from Arlington, Texas, was electrocuted when wiring problems in an apartment swimming pool’s underwater lights charged the water with electricity. A 16-year-old boy was seriously shocked when he jumped in the pool to try to save the young girl. Another teenager used a fiberglass shepherd’s hook (a non-conductive device) to pull both victims from the water. '

Your question appears to be about the danger from stray currents to swimmers.

This is the source story

The owner needs to sort out his electrics. The marina are contributing to the risk by continuing to supply a faulty installation.

 

[john_morris_uk]

[ QUOTE ]
[ QUOTE ]
Galvanic Isolator prevents small stray currents by requiring about 0.7 Volts to overcome the forward breakdown voltage of diodes. The diodes are wired in such a way that they don't (cannot) rectify the AC voltage, but do allow higher currents to pass

[/ QUOTE ]

Thats almost right but not quite. Galvanic isolators block current from very low voltage sources (actually up to about double the figure you quote) but allow current from higher voltages sources (Eg mains electricity) to pass so that the effectivness of the earthing sytem and is maintained.

[/ QUOTE ]Vic, perhaps you have phrased it better than me - tis what I was trying to say/thought I had said. I had SWMBO breathing down my neck saying 'we have to go NOW' as I was posting and couldn't re-read what I had written to see if it made sense!

 

[nedmin]

If what he says is true,which I doubt, its 1mA today but what about tomorrow! it could be carbonisation of a component which will get worse, as others suggest I would have strong words with the marina.

 

[misterg]

[ QUOTE ]
is 1 mA to 200mA such a big step?

[/ QUOTE ]

Yup.

(diagonal lines are current)

Taken from here which is quite an interesting read.

I concurr with others that the leakage will be more than 1ma to trip RCDs. I've seen RCDs available in 10ma (really sensitive), 30ma (most(?) houses), 100ma - in distribution systems which feed other circuits protected by lower rated RCDs, and I believe 300ma are available which is getting a bit scary!

Are you sure he is tripping out on earth leakage, not current overload??

I would also be interested in how he's arrived at a figure for the earth leakage.

My 0.02p

Andy

 

[Gerry]

Many thanks to you all for such informative replies.

There are a couple of details I left out of my email because I did not want to load the question too strongly.

There is not one but three boats on this pontoon with earth leakage faults. I know because I overheard the electricians shaking their heads about it when they came to find out why the alarms were going off. The manager of the marina has brushed their (and my) concerns aside. This might be because one of the boats that is leaking is his.

I did not say anything, until I saw a diver in the water just by his boat. Whereupon I unplugged his boat and called him up on his mobile with my concerns about the leakage risk. His first question to me was how did I learn about the problem!

He repeated that the earth leakage was only 1mA.

What should I do now? Email his boss, the owner of the marina? Name and shame the marina?

What do you suggest? Or am I just being paranoid?

Also, any thoughts on what leakage current begins to present a corrosion problem for a nearby (possibly aluminium) boat?

 

[2Tizwoz]

If you believe that the marina is being mismanaged in this way your best recourse would be to contact the local regulatory authority in writing giving them all your available information so that they are in a position to investigate and then dismiss or act upon your concerns.

If you are able to discover the tripping current on the RCDs and find that it is 10mA which gives a degree of personal protection you may rest more easily.

 

[Norman_E]

I had a further thought on this. If someone measured the earth leakage current they must have done so before the RCD tripped out, because afterwards there would have been no current. Therefore we cannot know what leakage current is actually causing the RCD to trip but it must be above the RCD trip point. The boats concerned need their elictrical systems sorting out as something on board has a serious and probably intermittent fault.
In a marina, it is not mains leakage which is the main cause of galvanic corrosion. It is the fact that all boats with shore power are connected together via the earth wires in the power cable. The earth wires are also bonded to metal fittings. The result is that the marina becomes one giant galvanic battery in which an aluminium boat with inadequate anodes could well suffer. The seawater in this case is merely the electrolyte, with bronze propellers and the like acting as cathodes. I am not sure what effect mains to earth leakages have in this except that metal fittings below water on the boat with the leakage must be at risk.

 

[srp]

Might be an idea to bang off a letter to the marina owner and copy it to, maybe, HSE and local electricity supply company, just to cover yourself, (esp if you or someone else has to make an insurance claim related to this in the future).
I agree with the others about the 1mA - that's obviously a load of BS. If you know where the RCD is located,and have access to it, it will say on the label what the trip current is.

 

[qetoo]

I think one of the problems causing earth currents is the marina wiring itself. In my marina I have disconnected the power cord and measured 7-volts AC between the shore power earth circuit (green wire) and the corresponding connection on my boat. Since the earth connection aboard my boat was connected to the engine block, this much voltage on the earth wire must have been driving substantial current into the surrounding waters. Unfortunately, I didn't have an AC ammeter to actually measure the current. Obviously a galvanic isolator could not block this much voltage.

The standard here in North America is 120 volts between the hot wire (black) and the neutral wire (white). Normally there should be near zero volts between the neutral wire and the earth wire. However, because in a marina electrical installation there is considerable distance between the shore-side electrical distribution panel and the boat receptical, it's not unlikely that there is maybe 10 volts or so voltage drop on the two power wires. So it would not be unusual to expect up to 10 volts different between the neutral wire and the earth wire out at the sub-distribution panel near the boat. However, most such sub-distribution panels I've seen do not separate the earth wires (green) and the neutral wires (white) within this panel. They both connect to the same ground-connection busbar. Thus the green earth circuit can actually be maybe 10 volts above actual ground voltage where the boat is afloat. Now if you connect the green earth circuit to your engine ground, you can be electrolyzing the surrounding waters by that much voltage.

[In the shore-side electrical panel, the neutral and earth circuits are tied together at the ground-bus with a substantial connection to a ground stake. Then out on the docks there are distribution sub-panels, but since this location is floating, there is no way to have a substantial ground stake. Most sub-panels that I've seen, tie the neutral circuit again to the earth circuit. Now, whatever voltage drop occurs normally between the shore panel and the dockside sub-panel, will produce some voltage between the earth wire and the actual earth where the boat floats.]

The best way of isolating against this is probably by using an isolation transformer on the boat. Of course, earth leakage detectors will also provide protection against this, but I have never seen one in any of the marinas around here. I have actually made an earth leakage detector installed into a short section of power cord. I have checked my boat with it as well as the boats in the adjacent slips.

 

[Jean]

Currently, Im not aware of any requirement for boat mains electrcs to be periodically tested, but with all the extra mains kit that can be wired in these days, well worth considering. Orignal post says "earth leakage detector", but more likely to be an RCD these days (they work differently). Also, worth asking the "faulty" boat owner just how he can be sure his leakage currant is only 1 mA.

 

[William_H]

As already questioned the owners claim that the earth leakage is only one milliamp is quite doubtful.
One typical form of protection is sometimes known as core balance relay. Here current in the active wire is compared to the current in the neutral any difference is presumed to be going through earth and may be a human path so it disconnects. The device is tested by connecting a known resistor from active O/p to earth so providing a known unbalance current. This varies but 15 ma or so is a ball park figure.
Unfortunately some equipment exhibits a fault which unbalances the currents and so causes the safety device to trip. This is usually more anoying than dangerous as the earth cable should dissipate the leakage current..

Your primary concern is that your power is being cut by a safety device which you share with other boats. You should not have to share as one device per boat would be far better and is more common.
Certainly if your neighbour plugs his boat into the common circuit and the safety device trips then he has a problem which is causing you to lose power.
it is very unlikely that this earth leakage current coulld ever do you any harm while in the water cleaning the hull etc. If it were a problem it would be only when you touched your prop which is connected to the mains earth via your power cable. So to be really safe disconnect the cable from your boat. Or more practically touch the prop initially with the back of your hand.

A stated a galvanic isolator stops current from the earth of your power system (which can be every one elses earth as well) flowing to earth via the prop etc unless the voltage rises to what might be more like fault voltage ie above 1.4 volts. It is unlikely that high AC currents will cause electrolysis as the current is reversed 50 times per second. It is small stray DC currents that cause problems. These are usually gnerated by dissimilar metals but the effect can be made much worse by small DC currents which hopefully the galvanic isolator will stop. (The whole earewa of corrosion and current paths is very difficult to sort out)

 

[Jean]

William, here in the UK, the "core balance relay" is known as an RCD (residual current device") and because it is so common in domestic properties, there is a standard "off the shelf" bit of these kit for testing it in the manner that you refer to. The same kit also effectivelly fully tests the installation that is being supplied. Recent (Jan 2005) Government regulations ("part P") have enabled instrument companies to capitalise on this and packaged the test kit as a "multifunction tester" known as "16th edition tester"; I have one, a great piece of kit. Marina electrics is not yet effectively fully covered by "part P" (but new "reg's" due out next year may change that), although, with pleasure boats not requiring formal testing (yet), I guess it's up to the marina operators to police it. I agree, each berth supply should ideally have its own RCD but, for economy, here they tend to share them with a few other berths.

 

[johnphilip]

Why do we need to connect our boats to the mains earth at all? I know the basic safety argument, but RCD protection does not require earth connection. Think of the number of appliances on the domestic and garden market which only have 2 wire connection, can we not treat the boat as such thus removing the problem (which I have suffered ) of fast erosion of anodes due I believe to stray earth voltages in the marina. Both the marina and my boat have RCD protection. Do we need more?

 

[Jean]

Glad to hear that you have your own RCD on your boat, it’s a requirement already for caravans and motorhomes, but yet to see it mandatory for leisure boats. The 2 wire appliances that you refer to are known as “class 2” and do not rely on basic insulation alone (eg, may be double insulated or even isolated via a transformer). For the RCD to function, the “fault current” needs a path to earth, either a deliberate earth or perhaps an accidental one. You can only treat your boat as “class 2” if your installation is so constructed and this is not normally the case. As for mains ac contributing to “erosion of anodes”, see earlier posts.