AC earth practicalities.

[ithet]

OK, I have fitted a shorepower system (using a garage CU) and want to connect the AC earth to DC ground

I have a forward domestic battery and the starter battery is aft near engine. The domestic battery earth is connected to the starter battery which in turn is earthed to the engine block. The DC panel earth runs to the same point on the engine.

I have some questions:

Is it required for the AC earth to run from the CU to the common earth point on the engine? It would be easiest to take it from a 13amp socket to the adjacent domestic battery earth, would this be safe enough?

What is the recommend wire type and gauge for this, obviously solid domestic earth is no use?


I am NOT asking whether to do this or not (I have considered the arguments for and against on board AC earthing).

Thanks in advance.

 

[Otter]

Unless your engine has an electrical path to the sea then what function does connecting the AC ground to DC neutral achieve?

 

[Stemar]

I've no knowledge of regulations, but I think I'd be reluctant to connect the earth to the engine block unless I could be absolutely sure the earth connection to the mains was always going to be perfect. I'd hate the thought of a corroded contact on the pontoon and a fault on the boat bringing the engine up to mains potential!

I'm sure someone will be along in a minute to tell me why it's a bad idea, but I've always fancied having the mains connection via an isolating transformer and doing away with the earth altogether.

 

[Puggy]

OK, I have fitted a shorepower system (using a garage CU) and want to connect the AC earth to DC ground

Is it required for the AC earth to run from the CU to the common earth point on the engine? It would be easiest to take it from a 13amp socket to the adjacent domestic battery earth, would this be safe enough?

Thanks in advance.

My answer is yes this is fine (i.e. you can take from a 13amp socket) as long as all the AC earths are tied together. This is broadly what I have installed. the AC earth is tied to the negative side of the batteries and then back to the engine block via one large piece of DC cable. The whole lot is is tied together near the CU and 12V DC panels. I also have a Galv Isolator between the boat and AC earth system shoreside.

 

[VicS]

I'm sure someone will be along in a minute to tell me why it's a bad idea, but I've always fancied having the mains connection via an isolating transformer and doing away with the earth altogether.

On the contrary, this is the best idea if you can accommodate an isolation transformer

You still create an on board equivalent of the earth with one side of the transformer secondary bonded to it in order to re-establish the safety aspects of an earth system.

By having no connection to the shoreside earth you also eliminate all the galvanic corrosion problems that can go with it.

Perhaps the only improvement would be to have the transformer on the pontoon rather than on the boat.

ISO 13297 is the International standard for AC systems on small craft. The latest edition is 2014 I believe but I have get to find it as free download

 

[ithet]

Thanks for comments so far. Can anyone help with the wire sizing?

 

[VicS]

Thanks for comments so far. Can anyone help with the wire sizing?

You are, I hope, wiring your AC circuits with flexible cable , not the solid conductor "twin and earth" used for house wiring.

I would use the same size ( cross-sectional area) and construction as that used for the plug socket circuit wiring

You should be consulting the European standard ISO 13297:2014 Small craft -- Electrical systems -- Alternating Current Installations and ensuring your system complies with its requirements.


I think, but its up to you to read the document thoroughly for yourself, that the wiring for your 16amp circuit should be 2.5mm² x 19 strand minimum. This is therefore what I would use for the earth bonding too.

Personally I would bond to the DC system directly from the CU rather that form the nearest plug socket

However
If David2452 joins the discussion take note of his advice .... it will be the correct advice.

 

[David2452]

Personally I would run the PE direct from the Consumer unit ising a gauge of at least that used in the live and neutral conductors (generally 2.5mm2f for a small craft install using 16a supply) and run it to the common ground for the DC circuit. I assume you have a GI installed in line before the Consumer unit and the advice also assumes that there is no generator or inverter involved.

 

[ithet]

Thanks David2452 and all. I shall try to run it like you say although it will be a longer run. No generator or inverter in my setup. Also I have no GI but am not connected to a permanent supply.

 

[pandos]

My answer is yes this is fine (i.e. you can take from a 13amp socket) as long as all the AC earths are tied together. This is broadly what I have installed. the AC earth is tied to the negative side of the batteries and then back to the engine block via one large piece of DC cable. The whole lot is is tied together near the CU and 12V DC panels. I also have a Galv Isolator between the boat and AC earth system shoreside.

+1 but make sure the galvanic isolator is a good type, lots of articles around about their reaction times and ability to withstand faults before blowing wide-open and leaving you with no earth to trip the ELCB so that you are left with a fault that will remain until the current blows the individual trip switch.

Good practice is to push the elcb test button when you connect to a new mains supply, this will give you an indication that the pontoon is likely to be ok.

use or ignore this advice entirely at your own risk.

Tony.