AC + DC Bonding

cagey

cagey

Well-Known Member
Joined
6 May 2004
Messages
2,241
Location
Cornwall
Visit site
Can't find a definitive answer: should AC shore power incoming 240v mains earth be bonded to DC neutral(ground) or not. Please also give reasons as all the research I have done gives sound reasons either way.
PS I understand wood and building not this strange leccy business, the local marine sparks also are confused.
K
 
cagey said:
Can't find a definitive answer: should AC shore power incoming 240v mains earth be bonded to DC neutral(ground) or not. Please also give reasons as all the research I have done gives sound reasons either way.
PS I understand wood and building not this strange leccy business, the local marine sparks also are confused.
K
Click to expand...

Defiantly keep separate
do a forum search, its been discussed before
 
search

took sailormans advice and searched using his name and found the post i think he was refering to. now i am even more confused as the consensus was at best evenly divided.
K
 
It stops any chance of galvanic corrosion if you have it separate and as long as you have an earth leakage trip, it is safe to do so.

The earth is either floating as for a generator or connected to the incoming AC shore power.

An earth leakage trip works by detecting any difference between the live and neutral so an earth connection is not involved.
 
The current wisdom among experts in the field is that the shorepower earth should be bonded to the DC negative and indeed the international standard ISO 13297 says so too but with the provision for it not to be provided there is a "whole craft" RCD.

I am advised that this provision will almost certainly be dropped from the next edition.

If bonded it becomes virtually essential for a galvanic isolator to also be fitted if the shorepower is left connected ( even if not actually in use ) for prolonged periods.

4 General requirements
4.1 The protective conductor insulation shall be green or green with a yellow stripe. Neither colour shall be used
for current-carrying conductors.
NOTE The equipotential bonding conductor of the d.c. electrical system (see ISO 10133) also uses green, or green with a
yellow stripe, insulation and is connected to various exposed conductive parts of direct-current electrical devices, other
extraneous conductive parts and the d.c. negative ground/earth.

4.2 The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as
practicable to the battery (d.c.) negative terminal.
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the
a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground
(protective conductor).

4.3 For craft with fully insulated d.c. systems (see ISO 10133), the a.c. protective conductor shall be connected
to the hull of a metallic hull craft, the craft external ground (earth) or the craft lightning-protection ground plate, if
fitted.

4.4 Metallic craft hulls shall not be used as conductors.

4.5 The protective conductor shall be connected to metallic hulls at a location above any anticipated water
accumulation.

4.6 Individual circuits shall not be capable of being energized by more than one source of electrical power at a
time. Each shore-power inlet, generator or inverter is a separate source of electrical power. The transfer from one
power-source circuit to another shall be made by a means which opens all current-carrying conductors, live and
neutral, before closing the other source circuit, prevents arc-over between contacts and is interlocked by
mechanical or electromechanical means. Both current-carrying conductors, live and neutral, shall be broken
simultaneously when changing power sources.

4.7 Energized parts of electrical equipment shall be guarded against accidental contact by the use of enclosures
of at least IP 2X type, in accordance with IEC 60529, or other protective means which shall not be used for nonelectrical
equipment. Access to energized parts of the electrical system shall require the use of hand tools or have
a protection of at least IP 2X, unless otherwise specified. A suitable warning sign shall be displayed (see 5.2).

4.8 The neutral conductor shall be grounded (earthed) only at the source of power, i.e. at the onboard generator,
the secondary of the isolation or polarization transformer, or the shore-power connection. The shore-power neutral
shall be grounded through the shore-power cable and shall not be grounded on board the craft.

4.9 A galvanic isolator or other suitable device may be fitted in the protective conductor to resist imported stray
galvanic current flow while permitting the passage of a.c. current, if present. Galvanic isolators shall be designed to
withstand the application of power from a short-circuit test from a source capable of delivering 5 000 A r.m.s.
symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After
three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be
unchanged.​

Cannot get a more definitive answer than that!
 
Last edited:
Vic has given you the official answer which will no doubt be seconded by everyone from your insurance company to the local H&S officer. The risk is that you have a battery charger which fails internally and puts 240v out on the 12v bus, and if one bit of this is not connected to earth, you could get a shock from, for example, the engine controls.

What you do is your personal decision. No one can sensibly make a recommendation other than the official route for fear of you electrocuting yourself and your wife suing. But I can tell you what I do / have done in the past.

What I do on the current boat is to have earth and battery negative connected and when leaving the boat and wanting mains power on, I connect the incoming mains direct to (say) the dehumidifier rather than into the boats system. I dont have a galvanic isolator but I do have an RCD.

On the previous boat, I did not connect the two having spoken to Sterling who assured me there was no way that the battery charger could fail to put 240v on the 12 v system. Again I had an RCD. I wouldnt go this route again though I didnt have any problems with it over 7 years.

It's undoubtedly best to follow the official policy, connect the two and get a galvanic isolator is you wish to leave the mains connected 24/7 on the pontoon.
 
I think there is a lot of misunderstanding with regard to bonding and RCD

1) markdj. The earth line is THE MOST IMPORTANT PART OF A SYSTEM WHEN USING A RCD. You are right in the fact the the RCD trips when there is a difference in the live an neutral lines current but the different in the current must go some where and that can only be down the earth line and back to the neutral of the generator bypassing the RCD, so EARTH IS VERY IMPORTANT else the RCD WILL NOT WORK.

2) I read with interest points 4.3 to 4.5 which refer to "metallic hull craft" and I agree that the earth MUST be connected to the hull if the boat hull is metallic as my case with it being steel.

Does this mean that non metallic hulled boats do not need this setup ?

Now if the hull is non metallic as in most boats being GRP IMHO only the items that are mains powered and have a metal case or fixed to a metal part of the boat need any earth connection at all. As the hull is non conductive what is the point of connecting all the metal items that have no mains power near them to the earth line.

As for Galvanic protection on an AWB it may be desirable to connect all underwater metal items together and to the anodes but why connect the mains earth to it as well.

As for connection the DC negative to the mains earth of to the Galvanic protection circuit why this may be adding to the problem. The general view for metallic hulled boats is that the DC negative must NOT be connected to the hull to reduce the possibility of galvanic corrosion. But the mains earth must be as there is a very real possibility of the hull becoming live is an earth fault occurs.

IMHO AC/DC bonding MUST be looked at within the overall context of the boat the materials of construction and the type of mains equipment fitted. No one size fits all can apply in this case as well as galvanic corrosion setup.

One of the things discussed in the past on the fora is earthing of onboard generators/ inverters and I not that point 4.8 covers exactly what I have said in the past and with particular to the comment by markdj above and the misunderstanding of the function of the earth line when a RCD is in circuit and the importance of the earth /neutral connection when on board mains generators/inverters are included in the installation.
 
As you can see I propose no connection between mains earth and 12V system and I agree with Charles Sterling that there is no possibility of 240VAC being through a fault to the 12 VDC system as the only piece of equipment that has both systems inside is the battery charger and all battery chargers I know of include a mains isolation transformer on there input and if a short did occur it would be to the case and so the mains earth. Any increase voltage in the DC charging circuit would blow the charger output fuse/circuit breaker thus disconnection it from the 12 VDC circuit.

If the battery charger does not have a mains isolation transformer and an output fuse/circuit breaker it has no place on a boat of even the garage at home.
 
Rogershaw said:
1) markdj. The earth line is THE MOST IMPORTANT PART OF A SYSTEM WHEN USING A RCD. You are right in the fact the the RCD trips when there is a difference in the live an neutral lines current but the different in the current must go some where and that can only be down the earth line and back to the neutral of the generator bypassing the RCD, so EARTH IS VERY IMPORTANT else the RCD WILL NOT WORK.
Click to expand...

I always thought that the protective function of an RCD was to detect the first small current flowing through you and so switch everything off. Surely it has to work this way or how would it work at all with continental two wire supply systems.
 
still confused.com

not trying to be deliberately stupid or controversial but the answers appear to be divided again.
Should you bond AC/DC Neutrals or not.
yes or no
thanks
K
 
not trying to be deliberately stupid
Click to expand...
Not the AC neutral but the shore power earth and the DC negative.

Yes you should .. See the extract from ISO 13297 above high lighted in red! READ IT!

BUT as I said earlier that standard does allow them not to be bonded provided there is an RCD that protects the whole AC instalation ( not just ceratin parts of it.

I belive the ABYC standards and possibly European standards require them to be connected with no exception allowed.

At the end of the day it's your boat and if privately owned and used its up to you. There is nothing to compel you to comply with any standards or recommendations, except perhaps your insurance company, but if you don't and someone gets electrocuted as a result then you will be the one in deep sh it.
 
OK, My boat is wood. My DC system is two wire, the engines are not the negative return, they are insulated return, no connections with the outside world.
I have a generator which is bonded to the earth of the mains system and so is the shorepower and inverter, all the earths are bonded together.
I do have anodes of course and they are bonded to the shafts, engines, "P" brackets, rudder and seacocks, belt and braces really as all those fittings under the water are proper bronze, even the shafts.
Why then should I bond my mains earth lead to my negative on my batteries? I read the regs, but dont still understand the why, can anybody explain?
I can see bonding the earth to my anodes, thus allowing any earth leak to go to ground (sea) but nothing else.
Thanks.
 
OK back again.

I am with ccscott49 on this one and would not bond AC earth to DC negative in any case and on my steel boat the AC earth is bonded to the hull due to the fact that my hull material is conductive and thus could become live if a live to appliance case fault did occur. On GRP boats IMHO are different due to the fact that the hull material is not conductive.

Also in the metal boat it is considered that connecting the DC negative to the hull could cause additional corrosion if return currents start flowing in the hull its self.

Now Bosun Higgs yes the RCD is designed to trip before the current get to the point where it would kill a normal person but the current reguired to kill someone depends on the condition of the person and the way the current flows through that person. A fit young person could take a higher current that an older person with may be a heart condition.

My point about the importance of an earth line is that when a live line to appliance case fault occurs that the with a good mains earth connection the RCD would trip before and person touched the live case of the appliance so preventing a shock in the first place.

This is mainly where the case of the appliance is metal (conductive) where the case is plastic and a person can only come into contact with the plastic case an earth is not required

This all presupposes that the earth line is connected to the supply neutral (local generator , inverter or isolation transformer) with shore power that is done at the sub station but not on your local boat.

This earth / neutral connection in fact is what defines which line output from the supply as neutral.

One point about RCD is that if a person gets a shock by being inline with the live or the neutral connection the RCD WILL NOT TRIP, the RCD only trips where the person is between live and earth (ground) and only if a good earth (ground) connection is at the substation.

This also begs a question about the connection of an on board power source as to if the neutral / earth line connection needs also to the connected to ground ( or the sea in the case of a boat) in my case having a steel boat and having my earth connected to the hull of the boat this happens naturally but on a GRP boat not so but again is it really needed if all the cases of the metal cased appliances are connected to a common earth point either where the shore power is connected or the earth/neutral point of on board mains supply devices.

An interesting point about galvanic isolators is the it has been considered the th GI must be able to carry the full current of the fuse/ circuit breaker but where a RCD is in circuit the maximum current that can flow in earth line and this the GI is the trip current of the RCD so why must the GI be rated so high? If any one has a different view I would be pleased to hear it.

Vic I have read the part in red and I know you are only quoting the standards but I cannot see what the advantage if the AC earth to DC negative connection can give and I and not asking you to comment unless you know the reasons the statdard was written that way other that just convention. As I state above IMHO DC circuits and AC circuits should be separate and independent.

BTW the AC earth and neutral are connected together at the supply device as stated above so neutral will be connected to DC negative.
 
Last edited by a moderator:
Why then should I bond my mains earth lead to my negative on my batteries?
Click to expand...
I read the standard as saying than with a fully isolated DC system you dont! It would no longer be fully isolated if you did!
 
back again

because i charter my boat i applied the same logic as i have to in my main business (holiday park and residential flats). that is if there is a risk get a certificate.hence my question re linking earth and neutral. the question came about because the electricians in Falmouth,my charter co and my coding surveyor thought i was at best a nutter.
Today i have finally persuaded an electrician to test my installation and issue albeit on his letterhead a cert that my installation is safe and the rcd( could have been rccd)tripped within x . secs.
He thought i was a fool as he kept telling me there is no uk requirement, no iea certs and no way of getting a true mega value as the boat is it in water and covered by the marinas hook up but he was happy to take my money.
at least i am happy that the boat has been approved by a professional, but i still don't understand the huge differences in opinion.
K
 
VicS said:
I read the standard as saying than with a fully isolated DC system you dont! It would no longer be fully isolated if you did!
Click to expand...

Yep, sorry Vic I also read that, but didnt you say, that this would be changed or dropped in the new regs, or am I barking up the wrong conifer?
Anyway, thanks for your clarification, I will stick with the system I have, it appears to be safe (hopefully)!
 
Steel boat...

quote from "Also in the metal boat it is considered that connecting the DC negative to the hull could cause additional corrosion if return currents start flowing in the hull its self."


A ground from the DC negative battery terminal to a secure bolt welded to the hull is necessary in order to allow the circuit breakers to function.

(Givens: all circuits are tinned copper and have a positive and negative wires and each is protected with a suitably sized circuit breaker)
 
ianabc said:
quote from "Also in the metal boat it is considered that connecting the DC negative to the hull could cause additional corrosion if return currents start flowing in the hull its self."


A ground from the DC negative battery terminal to a secure bolt welded to the hull is necessary in order to allow the circuit breakers to function.

(Givens: all circuits are tinned copper and have a positive and negative wires and each is protected with a suitably sized circuit breaker)
Click to expand...

Roger has opted for a fully isolated DC system. He has no connection between the DC system and the steel hull. That is his choice and what he feels is the best thing to do. No connection means no chance of any DC currents flowing through the hull.

If the DC system in an metal hull is not isolated then there should only be one connection to the hull. The hull should not be used as a conductor in the way that a car body is used as the negative conductor.
Any circuit breakers in the DC system will system will work and trip on overload currents in exactly the same way regardless of whether the DC negative is grounded to the hull or not !
 
100% Vic

As you so correctly pointed if there is no connection between AC earth and DC negative the DC circuit must be isolated unless there is a connection between the DC negative and the interconnection of the metal fittings below the waterline.

In my case my engine is isolated from the hull and the prop shaft / stern gear is also isolated from the engine but electrically connected to the hull.

My anodes are bolted to the hull by welded studs and the prop shaft has its own anode.

I think in may be over protected hence my questions about suppliers of silver half cells which I may have found one at a good price from South east marine in good old US of A.

"A ground from the DC negative battery terminal to a secure bolt welded to the hull is necessary in order to allow the circuit breakers to function"

Would love to know where that statement comes from ? A Vic says totally wrong. May be some confusion with RCD's and the earth/neutral connection on AC systems.
 
You must log in or register to reply here.

Share this page

Top