Is my hull possibly live and not earthed?

[cheesegas]

15 minutes ago, Alan de Enfield said:

Until there is some form of recognition for 'skills' the inland waterway is wide open to anyone claiming to be anything,

There is the BMET qualification which aims to fix that, but it's only a two day course and the exam is stupidly easy. It only covers the very basics and nothing about proper system design, nor much else about how to install things properly.

 

To be fair they do suggest that the BMET is combined with another full qualification to get a proper education, but it's still possible to do a 2 day BMET course with little experience, call up and get liability insurance, perhaps trade under a limited company and go out and do electrical stuff on boats perfectly legally.

 

(one of the few things the BMET does however teach is that battery negative be tied to the hull as they teach to the BMEAA code which follows ISO13927!)

[JonA66]

Sorry to come back with another question. If I put my multimeter on the positive battery terminal and connect to the hull I should see 13v ish depending on how charged they are. However if I connect the negative to the hull I should get zero voltage is that correct. If I get voltage to negative that means the hull is live and there is a short somewhere. I don’t think this will be the case as the battery would have discharged in this circumstance?

[Paul C]

6 minutes ago, JonA66 said:

Sorry to come back with another question. If I put my multimeter on the positive battery terminal and connect to the hull I should see 13v ish depending on how charged they are. However if I connect the negative to the hull I should get zero voltage is that correct. If I get voltage to negative that means the hull is live and there is a short somewhere. I don’t think this will be the case as the battery would have discharged in this circumstance?

 

Sorry to not answer the question, but it needs clarification. Firstly, when you say multimeter, do you mean a multimeter set to read voltage? Or something else? Because its a multimeter........it can read resistance and current (and probably a few more things) too. Its worth clarifying, although I'm 99% sure you do mean DC voltage. But never assume....

 

Secondly, a voltmeter reads potential difference between 2 points. So when you say "connect the negative to the hull" what do you mean? The negative (black) lead of the multimeter? Where is the other lead connected - the hull too? The negative battery terminal? etc.

 

Also worth mentioning, the (thick) cable that connects the battery negative to the hull doesn't carry any current in normal operation. Its only in fault conditions it would carry current. The idea being, that if a wiring fault occurred which let a positive wire touch the hull, instead of raising the potential of the hull it would flow a current which would trip a circuit breaker or maybe blow a fuse if fitted. Yes in normal non-fault conditions, you'd want to see 0.000V between the battery negative and the hull.

[Tony Brooks]

18 minutes ago, JonA66 said:

Sorry to come back with another question. If I put my multimeter on the positive battery terminal and connect to the hull I should see 13v ish depending on how charged they are. However if I connect the negative to the hull I should get zero voltage is that correct. If I get voltage to negative that means the hull is live and there is a short somewhere. I don’t think this will be the case as the battery would have discharged in this circumstance?

 

As Paul points out, these tests have not been defined clearly enough.

 

Assuming the meter is set to volts - test 1, bat positive to hull, yes battery voltage would be expected, whatever that is.

 

Test 2 and assuming that you have one meter lead on the battery negative and the other on the hull. This should read zero, but if you find voltage, it means that somehow the hull has become live. It also means that the negative bond to the hull is faulty, otherwise a fuse would have blown, or a cable would catch fire. I agree that if left, it would allow the batteries to discharge themselves, but note that you often find a very few mV caused by goodness knows what. In theory, you should not see any voltage. I suspect those few mV may be "noise" in the meter electronics.

[JonA66]

Sorry yes both of you are correct - multimeter set to dc voltage, connect red wire to battery negative and black wire to hull so this should read zero or very minor "noise" voltage 

 

[PeterF]

3 hours ago, JonA66 said:

Sorry yes both of you are correct - multimeter set to dc voltage, connect red wire to battery negative and black wire to hull so this should read zero or very minor "noise" voltage 

 

Correct.

[jonathanA]

I slightly disagree with the above. Yes you should get zero or a few millivolts. However if there are some poor connections or  other faults in the negative leads and engine/hull/battery neg leads you might read .5v or more depending on what is switched on etc.... and it might be worth investigating further.  I had a problem a while ago where I was losing about half a volt between the alternator case and battery neg when charging the batts via the alternator. Turned out some bright spark had connected the engine neg bond to an engine mounting bolt. Which would have been fine if they had put it on the engine side rather than the engine bed side where the rubber mounts did a good job of getting in the way of the electrons...

 

I think its unhelpful to say a reading means the Hull has become 'live' as that may not be the case at all.

 

I don't think the OP should get too concerned as I think the concensus is the mechanic didn't have a Scooby on electrics...

 

[Paul C]

4 minutes ago, jonathanA said:

I slightly disagree with the above. Yes you should get zero or a few millivolts. However if there are some poor connections or  other faults in the negative leads and engine/hull/battery neg leads you might read .5v or more depending on what is switched on etc.... and it might be worth investigating further.  I had a problem a while ago where I was losing about half a volt between the alternator case and battery neg when charging the batts via the alternator. Turned out some bright spark had connected the engine neg bond to an engine mounting bolt. Which would have been fine if they had put it on the engine side rather than the engine bed side where the rubber mounts did a good job of getting in the way of the electrons...

 

I think its unhelpful to say a reading means the Hull has become 'live' as that may not be the case at all.

 

I don't think the OP should get too concerned as I think the concensus is the mechanic didn't have a Scooby on electrics...

 

 

But that would be a voltage drop between nominally grounded (negative) points which would always been seen, to a greater or lesser extent, in normal use.

 

We're talking about the fault condition of a current flow between battery negative and hull, which should never occur.

[jonathanA]

2 minutes ago, Paul C said:

 

 

We're talking about the fault condition of a current flow between battery negative and hull, which should never occur.

No the op was talking about measuring the PD between the hull and the neg batt post. One reason for a reading could be a fault resulting in a current in the hull, eg someone using the hull for the return path for something. I'm saying there are other possibilities. Which I think you agree/accept.  

[David Mack]

10 minutes ago, jonathanA said:

However if there are some poor connections or  other faults in the negative leads and engine/hull/battery neg leads you might read .5v or more depending on what is switched on etc....

If you are getting a volt difference of 0.5V between the engine negative lead and the hull, then you must have some significant current flowing through the earth strap, and that can only happen if some appliance somewhere in the boat has its negative connected to the hull, rather than having a wired negative connection back to the battery. This can arise if you use appliances intended for vehicles which are designed for the return current to flow via the vehicle body. Examples could be headlight, horn, car radio. If you use such appliances, you need to isolate the appliance body from the shell by an insulating material (e.g. plastic) and arrange that there are no direct bolted connections (unless you use plastic nuts and bolts), and with a negative lead back to the battery connected to the appliance body (e.g. crimped ring connector under a fixing bolt).

[jonathanA]

Yes in fairness in my example wasn't actually  measuring hull to batt neg and I agree you would need a fair old current to see something across the hull bond, Whichwould typically be a couple of feet of 25mm cable...  My point was that you need to careful in measuring a voltages, expecting zero and then getting a reading and jumping to conclusions. 

[MtB]

52 minutes ago, jonathanA said:

No the op was talking about measuring the PD between the hull and the neg batt post.

 

Are we?

 

From the thread title I was thinking one probe on the multimeter on the hull, the other stuck into the bank or pontoon to measure the extent to which the hull was "live" (whatever that means).

[magnetman]

Earthing in Boats is not complicated. 

 

Just take a lead from the battery negative, add a spike and put it into one of the geranium pots carried on top of the cabin. 

[Alan de Enfield]

4 minutes ago, magnetman said:

Earthing in Boats is not complicated. 

 

Just take a lead from the battery negative, add a spike and put it into one of the geranium pots carried on top of the cabin. 

 

Would a Busy Lizzy do ?

[Paul C]

1 hour ago, jonathanA said:

No the op was talking about measuring the PD between the hull and the neg batt post. One reason for a reading could be a fault resulting in a current in the hull, eg someone using the hull for the return path for something. I'm saying there are other possibilities. Which I think you agree/accept.  

Well.....no. There should never (in normal use) be any current from hull to negative terminal of the battery. The bonding is done to ensure eg a circuit breaker pops in a fault condition rather than raising the potential of the hull.

 

If the hull is being used as a return path, it either indicates a faulty installation of something, or a stray connection by something else to the hull (which is also a fault). 

[JonA66]

Hello all. Thanks for the further uodates. The reading was zero so I think all is fine