Earth bonding

[Theo]

I fully accept the following principles and hold them to be incontrovertible:

 

1. That the neutral and earth on the inverter must be bonded to the hull.

2. That the neutral and earth on the boat side of an isolating transformer must be bonded to the hull.

3. That the negative of the 12V system must be banded to the hull.

4. That all of them need to be bonded to the hull at the same point.

 

What I am less certain about is what to do with the other hull/engine bonding points.

 

Our BMC 1.5 is fitted with a 100A Bosch alternator. It has th B+ terminal to which I have attached a 35mm^2 cable which runs to the split charge relay and thence to the battery bank. The negative of the battery bank is connected to the engine block. This means that the negative return to the alternator for the starter motor runs through the engine block. It also means that there are two earth bonding points: the bolt on the engine and the earth bonding point to the hull for the things mentioned in the list of principles above.

 

It seems to me that there is a path for stray currents between the two earthing points and hence the possibility of corrosion.

 

Here are the questions:

 

Is the engine likely to be properly bonded to the hull so that I can remove the separate earth bonding point and instead use the one on the engine? Should I not worry?

 

Nick

Edited May 23, 2009 by Theo
Spelling, typos and paragraphing

[Gary Peacock]

Your setup is pretty much normal don't worry about it.

[Gibbo]

There's nowt wrong with it. Go and have a beer.

 

Problems can occurr if the bonding points are some distance apart (say on either side of a hull - but even that would be very unlikely to do any damage) so that there is actually some measurable resistance between them to cause a voltage difference. It would take a few tens of thousands of amps to get a voltage difference across an engine block.

 

Gibbo

[Theo]

There's nowt wrong with it. Go and have a beer.

 

Problems can occurr if the bonding points are some distance apart (say on either side of a hull - but even that would be very unlikely to do any damage) so that there is actually some measurable resistance between them to cause a voltage difference. It would take a few tens of thousands of amps to get a voltage difference across an engine block.

 

Gibbo

 

I have one small bonding point on the cross bulkhead near to the negative busbar and one on the engine block. OK?

 

N

[chris w]

Is the engine likely to be properly bonded to the hull so that I can remove the separate earth bonding point and instead use the one on the engine? Should I not worry?

 

Nick

The only concern in this regard is that the engine IS actually bonded to the hull and is not sitting on rubber bushes with earthing being (inadvertently) achieved through the throttle and gear cables. They may not make a great fuse!!

 

Chris

[Gibbo]

They may not make a great fuse!!

 

 

On the contrary. They make an excellent fuse for the starter motor.

 

Gibbo

[NB Alnwick]

 

On the contrary. They make an excellent fuse for the starter motor.

 

Gibbo

 

In my experience proper marine engine electrical equipment (i.e. Starter Motor, Alternator) will have both positive and negative connections that are independent. Nevertheless, it seems to be a wise precaution to make sure that the engine is bonded to the hull and if flexible engine mounts are used, then this may require at least two flexible 'earth straps' as used on commercial and military vehicle engines.

Edited May 24, 2009 by NB Alnwick

[chris w]

Nevertheless, it seems to be a wise precaution to make sure that the engine is bonded to the hull and if flexible engine mounts are used, then this may require at least two flexible 'earth straps' as used on commercial and military vehicle engines.

Just run a thick earth cable from a bolt on the engine to the hull bonding point. Simples

 

Chris

[Theo]

Just run a thick earth cable from a bolt on the engine to the hull bonding point. Simples

 

Chris

 

That's the sort of answer that I wanted to hear. I can do that easily. I have a longish loan on the hydraulic crimper and can sor that out next week.

 

Many thanks for all the advice.

 

Nick