Jump to content

Bonding / Earth


Featured Posts

Evening ladies and gents - another electrical quandry.

This probably wouldnt be a quandry for someone who had half a clue. I have read Terry Brooks and Smartgauge a fair bit, whilst much of it goes over my head I have learnt that bonding both AC and DC is important, particularly AC. Despite this, many people tell me I'm crazy to want to earth the boat, there's obviously a lot of confusion around the subject.

Currently, fit out in process, my AC comes in via shoreline (with earth running through the GI) and straight into the Victron inverter/charger. I haven't started the 12v distribution/circuits yet. I have an MPPT, Inverter and 230vac consumer unit that all need earthing and when I start wiring 230vac sockets, they'll have an earth too of course. That's all fine and good when Im plugged into shoreline, as the earth runs back to the shore. However, when I'm crusing, what happens?

  1. Should I run an earth from the GI to the engine or hull? - to account for safety when crusing.
  2. Can I run the aforementioned devices to a stud (common earth) then back to the GI? - to save on holes through the bulkhead
  3. To earth the DC should I wire from the last battery (negative return) to the engine or hull?

Many thanks in advance,

Link to comment
Share on other sites

When you are cruising, the Combi should (if set up correctly) have a relay connect its neutral to earth so that the RCD will still work. Called an NE bond or suchlike.

1) Hull, not engine

2) In order for RCDs to work, the hull must be earthed. You can run an earth wire from the boat side of the GI to the Combi and then on to the consumer unit and then on to the services you mention via normal 3 core flex. Exactly where you choose to make the connection between earth circuit and the hull doesn't matter too much, probably at the consumer unit is best but at the GI would also be OK. Better not to run the earth via the hull stud though, as this increases the probability of an earth circuit disconnect (long run of wires somewhere in the depths of the rusty bilges!).

3) The engine needs to be well connected to the battery negative so that there is a circuit for the starter and alternator. This will to some extent also connect to the hull via various metal fittings etc but to do the job properly you would connect a cable between the engine and the hull. If you connect a cable from the battery -ve to the hull, this present another route for current to flow between battery -ve and engine, through the hull, which is a BAD THING.

 

 

Edited by nicknorman
Link to comment
Share on other sites

I'm not really sure why this subject causes so much confusion but I'll try to make it simple. 

With no Combi the incoming shore power goes to the consumer unit. That's all 3 wires, LNE. If you have a GI then the Earth wire from the shoreline input goes to the consumer unit via the GI. 

If you have a Combi then the incoming shore power goes to the Combi. That's all 3 wires, LNE. If you have a GI then the Earth wire from the shoreline input goes to the combi via the GI. The output of the Combi (all 3 wires, LNE) goes to the consumer unit. 

The consumer unit has a wire (at least 4mm2) from its earth busbar to the hull. 

That's it. It's that simple. 

Tony

Link to comment
Share on other sites

Cheers Nick & Tony, thanks for taking the time.

Nick, regarding the actual method for bonding, I was considering welding a stud directly onto the hull, up high in the engine room, close to the GI. Boat side of the GI, as you advise. Would this be suitable to cover the AC?

Then, regarding 3), my engine (Beta60) seems to have a stud on it which I can ground the DC to on the -ve. The engine is on mounts, which seem to have rubber feet. Are you suggesting that grounding to the engine is sufficient and I should weld another stud to the hull and connect the engine to that?

Link to comment
Share on other sites

16 minutes ago, Mohsen said:

Cheers Nick & Tony, thanks for taking the time.

Nick, regarding the actual method for bonding, I was considering welding a stud directly onto the hull, up high in the engine room, close to the GI. Boat side of the GI, as you advise. Would this be suitable to cover the AC?

Then, regarding 3), my engine (Beta60) seems to have a stud on it which I can ground the DC to on the -ve. The engine is on mounts, which seem to have rubber feet. Are you suggesting that grounding to the engine is sufficient and I should weld another stud to the hull and connect the engine to that?

Ideally you have two studs welded to the hull reasonably close to each other so that it's obvious to anyone looking that these are the bonding studs. 

One of the studs will carry the single mains Earth wire from the Consumer Unit. 

The other stud will have the two negatives from the batteries (leisure and starter) plus a bond to the engine. Or, if you've taken the battery negatives to the engine then this DC bond will only have one wire - the bond to the engine. 

Edited by WotEver
It was a mess!
Link to comment
Share on other sites

10 hours ago, WotEver said:

 

The other stud will have the two negatives from the batteries (leisure and starter) plus a bond to the engine.

I disagree on this point. If you connect the battery -ves to the hull, there is a current path between the battery -ve and the engine starter and alternator negatives (big currents!) through the hull and the inevitable metal connection between engine and hull (exhaust, control cables, maybe prop shaft /bearings). This is a bad thing! If you put a cable between the hull point where you attach these negatives and the engine, the current is then shared in inverse proportion to the relative resistances but there is still likely to be quit a big current flowing through part of the hull.

The correct way to do it is to take the battery negatives to the engine, and then have another wire going from there to the hull. That way there are no current flowing through the hull. This is why Beta provide 2 pairs of studs on the engine, one each for domestic battery -ve, engine battery -ve, domestic battery positive and engine battery positive

Edited by nicknorman
Link to comment
Share on other sites

1 minute ago, nicknorman said:

I disagree on this point. If you connect the battery -ves to the hull, there is a current path between the battery -ve and the engine starter and alternator negatives (big currents!) through the hull and the inevitable metal connection between engine and hull (exhaust, control cables, maybe prop shaft /bearings). This is a bad thing! If you put a cable between the hull point where you attach these negatives, the current is then shared in inverse proportion to the relative resistances but there is still likely to be quit a big current flowing through part of the hull.

The correct way to do it is to take the battery negatives to the engine, and then have another wire going from there to the hull. That way there are no current flowing through the hull. This is why Beta provide 2 pairs of studs on the engine, one each for domestic battery -ve, engine battery -ve, domestic battery positive and engine battery positive

A fair point well made. So as per the 2nd part of my last para:

10 hours ago, WotEver said:

Or, if you've taken the battery negatives to the engine then this DC bond will only have one wire - the bond to the engine. 

 

Link to comment
Share on other sites

4 minutes ago, nicknorman said:

I disagree on this point. If you connect the battery -ves to the hull, there is a current path between the battery -ve and the engine starter and alternator negatives (big currents!) through the hull and the inevitable metal connection between engine and hull (exhaust, control cables, maybe prop shaft /bearings). This is a bad thing! If you put a cable between the hull point where you attach these negatives and the engine, the current is then shared in inverse proportion to the relative resistances but there is still likely to be quit a big current flowing through part of the hull.

The correct way to do it is to take the battery negatives to the engine, and then have another wire going from there to the hull. That way there are no current flowing through the hull. This is why Beta provide 2 pairs of studs on the engine, one each for domestic battery -ve, engine battery -ve, domestic battery positive and engine battery positive

I was waiting for that :captain:

Link to comment
Share on other sites

So from the answers given. 

I need two studs, welded to the hull, sited well and labelled.

1) One stud connects to the GI (boat side) which earths the AC

2) Both battery bank's -ve connects to the engine. Then via a different part of engine, a cable is run to the second hull stud.

This keeps everyone safe and boat free of galvanic erosion ?

Link to comment
Share on other sites

1). The cable should come from the Consumer Unit, not directly from the GI. There are good reasons for that which I can go into if you wish. 

2). The bond from engine to hull doesn't need to come from a different point on the engine, it can come from the same place as the batt negs connect. 

  • Greenie 1
Link to comment
Share on other sites

Cheers Tony,

That surprises me. The primitive assumptions I've made. As the cable goes shoreline-> GI-> Combi -> Consumer unit

If the earth goes from the consumer unit to a stud on the hull doesn't it by-pass the Combi?

further, if the that current isn't passed through the GI before it's route to the earth stud, isn't there a chance it could create a scenario where galvanic erosion could occur?

Link to comment
Share on other sites

But it does pass through the GI before going to the bonding stud. You said yourself that it goes Shore Power --> GI --> Combi --> CU. So the shore earth is isolated by the GI as soon as it enters the boat. 

Link to comment
Share on other sites

Okay, the reasons for bonding it from the CU are twofold, the second being by far the most important.

1. It is where an electrician would expect to find it. 

2. If the Combi ever got replaced by a standalone inverter (even temporarily, say if the Combi went off for repair) then bonding the earth from the GI would mean that the inverter output would not now be bonded. 

Link to comment
Share on other sites

Not altogether sure I understand some of this so a couple of questions if I may:-

  1. Our boat has a connection from the inboard stud of the SafeShore GI to a hull mounted stud, and another, separate, connection from the casing of the Sterling Pro-Combi inverter / charger to the same hull mounted stud. Is this arrangement good, bad or indifferent?
  2. The boat has a Beta BV1505 fitted with a stud that has a -ve connection to all the batteries, the domestic and engine -ves being linked. Is this a sufficient DC 'hull bond'? Is the possibility of galvanic erosion decreased by fitting a new hull mounted stud and taking a cable from the -ve connection on the engine to the new stud?

Mohsen, apologies for 'borrowing' your thread!

Link to comment
Share on other sites

29 minutes ago, Col_T said:

Is there a way of cancelling a draft reply??

If it's just a draft then yes, just don't hit 'post'. If you've already posted a load of borrocks then all you can do is edit it. ;)

Link to comment
Share on other sites

On 09/08/2017 at 20:55, Mohsen said:

Evening ladies and gents - another electrical quandry.

This probably wouldnt be a quandry for someone who had half a clue. I have read Terry Brooks and Smartgauge a fair bit, whilst much of it goes over my head I have learnt that bonding both AC and DC is important, particularly AC. Despite this, many people tell me I'm crazy to want to earth the boat, there's obviously a lot of confusion around the subject.

Ask those people if they would be happy to use an old cast metal case power tool, in which the internal earth to the metal case had been removed? If anyone says "yes" then ask them if they would still be ok using the tool if part of the casing was touching the surface of a bucket of water?  

I'm no electrician but if you bring mains electricity onboard a floating steel box isn't that essentially what you're doing? You'd have to be crazy to do it without earthing the hull.

Edited by blackrose
Link to comment
Share on other sites

  • 2 weeks later...

One final question on this!

is it of any importance that the bonding/earth point on the hull is on a part that is always below the water line?

I ask because the bulkhead wall in the engine bay that separates the main cabin is the most logical place in terms of neat wiring. However this is neither below the waterline nor on an 'exterior wall'?

Link to comment
Share on other sites

If you do mean below the waterline then it's of absolutely zero significance. If you mean below water then that's bad news from a Corrosion standpoint, but I don't think you do mean that :)

Have the bonding point on any steel part of the hull, preferably adjacent to the -ve bonding point. 

Link to comment
Share on other sites

  • 2 weeks later...
11 minutes ago, Mohsen said:

Just for posterity. In case someone comes across this and finds it useful.

IMG_5263.JPG

It's generally held as best advice for safety on narrowboats to not share the DC bonding point with the AC bonding point but to have them close together. 

Link to comment
Share on other sites

6 minutes ago, WotEver said:

It's generally held as best advice for safety on narrowboats to not share the DC bonding point with the AC bonding point but to have them close together. 

Agreed, but they are separated for is practical reasons, (ease of identifying each earth point) rather than electrical reasons As the hull effectively commons the two earth points.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.