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Why Do I get Sparks?


Rebotco

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When connecting a new bilge pump direct to the batteries, my spanner accidentally touched the the hull whilst loosening the positive battery terminal.  This resulted in a small, but very definite spark, which puzzles me.  I didn't think there was any contact between batteries and hull, so why would I get a spark?  

I do not have any shore power circuits (do have a small inverter though).

Does this spark indicate something might be wrong?  Any enlightenment would be appreciated.

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You completed a circuit between the positive battery terminal and the hull. The bate try negative should be connected to the hull in one place.  There is much argument about this but its why I advocate ensuring EVERYTHING is turned off and disconnecting the negative lead first. You are very lcky you did not end up with the markings on the spanner branded into your hand!

  • Greenie 2
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Any spanner used on battery terminals should be completely insulated except for the jaws. ALWAYS!  I have one spanner reserved for the purpose covered in insulating tape. It is worth the cost of an extra spanner. At the battery posts you have no fuses to limit the damage.  One slip with a bare spanner and you can be doing welding with bare hands.

Parhaps the availability of an insulated battery spanner next to the batteries should be a BSS requirement?  Dons insulated tin hat and heads for the bunker...

Added: I'm curious as to why you only got a small spark.

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14 minutes ago, Tony Brooks said:

You completed a circuit between the positive battery terminal and the hull. The bate try negative should be connected to the hull in one place.  There is much argument about this but its why I advocate ensuring EVERYTHING is turned off and disconnecting the negative lead first. You are very lcky you did not end up with the markings on the spanner branded into your hand!

Thanks - I was not aware of that!  Is this to do with the engine starter battery?

If the hull is a negative, why cannot boats be wired with one positive cable to appliances, with the return via the hull, like in cars?

Sorry if these are dumb questions.

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The thing to do, as Tony says, is to always fully disconnect the negatives before doing battle with the positives.  Stick to that rule, and you will never have any risk of pyrotechnics when spannering the positive connections.

 

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21 minutes ago, Rebotco said:

Thanks - I was not aware of that!  Is this to do with the engine starter battery?

If the hull is a negative, why cannot boats be wired with one positive cable to appliances, with the return via the hull, like in cars?

Sorry if these are dumb questions.

Not dumb questions.  the dumb question is the one that wasn't asked resulting in problems.

Negative returns through the hull are best avoided even though it looks like a cost saving solution.

It is possible that they may case hull corrosion.   People with better explanations will be along soon to put flesh on the bones but 'mean time do as Tony says.

 

Edited by Taslim
WRONG BUTTON....
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Presumably the negative is connected to the engine block, therefore any pos. stray wire can make the circuit. I do wonder why there isn't some kind of two pole isolator arrangement as a safety requirement for engine starting. Seems sensible to me but electrics are a bit of a mystery to me.

Edited by Bee
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25 minutes ago, Chris J said:

I've noticed that I get a spark when I tighten by final negative terminal back up after taking the batteries out for doing any maintenance. Anyone know why this is how it can be avoided?

Do you have the isolator switched off? If not then do so. If you do then something that draws current is bypassing it. 

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10 hours ago, Bee said:

Presumably the negative is connected to the engine block, therefore any pos. stray wire can make the circuit. I do wonder why there isn't some kind of two pole isolator arrangement as a safety requirement for engine starting. Seems sensible to me but electrics are a bit of a mystery to me.

Probably are on ships BUT for amateur use a fault in the switch plus a fault elsewhere could result in the hull becoming live with the associated corrosion risks. By having the hull bonded to battery negative you ensure that in the event of a positive short to the hull the fuse or circuit breaker operates.

8 hours ago, Chris J said:

I've noticed that I get a spark when I tighten by final negative terminal back up after taking the batteries out for doing any maintenance. Anyone know why this is how it can be avoided?

If you have anything with capacitors in then that could be the capacitors charging or possibly discharging. More likely as Tony says there is something still connected and drawing or delivering current. Solar?

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8 minutes ago, rasputin said:

Why the unpleasantness? 

Sorry, that was me trying to be funny, no offence intended. The "desperate" solution to the OPs problem is to get a boat that didn't have an electrically  conductive hull., i.e. wood or grp.

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it concerns me more than slightly that a boater who has been on here for more than 7 years and considers himself competent to wire stuff up was not aware that the hull is connected to the negative (earth).

how on earth has he managed so far?

there are so many books and internet links explaining how steel canal boats work and, even with 50 years general boating experience under my hat, I bought a few books before I ventured into canal boating. ..... try The 12 Volt Bible for one.

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Yes, when you think about it, how has anyone who's been living on this planet for 4, 5 6 or 7 decades not managed to be aware of every bit of knowledge that's in the public domain?

Life is a learning process. Nobody can know everything and there are always bit and pieces of even basic information that can slip through the net.

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2 hours ago, Tony Brooks said:

If you have anything with capacitors in then that could be the capacitors charging or possibly discharging. More likely as Tony says there is something still connected and drawing or delivering current. Solar?

Even if it’s something with caps that ‘something’ must be connected to the positive side of the batts to complete the circuit. 

Possibilities that bypass the isolator are alternator, car radio, bilge pump, fridge, inverter/charger...

Take a look at the wiring - there should be one, and only one, fat cable from batt +ve to the isolator. If there are any other wires connected to the battery or that side of the isolator then one of those (or rather, what it feeds) is the culprit. 

2 hours ago, Tony Brooks said:

By having the hull bonded to battery negative you ensure that in the event of a positive short to the hull the fuse or circuit breaker operates.

And that is the main reason for bonding the hull. 

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2 hours ago, billh said:

The proper answer is to get a wooden boat with two pole  isolated alternator and starter. That way there's no point in any electrical connection to  the hull.

I suppose a GRP boat  would do the same  if you are really  desperate.:D

I am not sure this is a full answer because although it would mitigate hull corrosion there is still potential for short circuits to metal on most inland boats. The engine electrics are usually earth return even if the starter and alternator look as if they have insulated return negative connections. The instruments and glow plugs certainly are.  This means the control cables and control lever assembly are connected to negative as is often the shaft and stern gear. That leaves scope for short circuits etc.

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23 hours ago, system 4-50 said:

Any spanner used on battery terminals should be completely insulated except for the jaws. ALWAYS!  I have one spanner reserved for the purpose covered in insulating tape. It is worth the cost of an extra spanner. At the battery posts you have no fuses to limit the damage.  One slip with a bare spanner and you can be doing welding with bare hands.

Parhaps the availability of an insulated battery spanner next to the batteries should be a BSS requirement?  Dons insulated tin hat and heads for the bunker...

Added: I'm curious as to why you only got a small spark.

Well well. Forty years a sparkie and never seen this before. Most of us pros just make do with doing the job correctly and disconnect the earthed terminal first and re connect it last. Zero chance of a spark if you have any real idea what you are doing.

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1 hour ago, Sir Nibble said:

Well well. Forty years a sparkie and never seen this before. Most of us pros just make do with doing the job correctly and disconnect the earthed terminal first and re connect it last. Zero chance of a spark if you have any real idea what you are doing.

Ok. You know that and don't need to be told anything. I'm just trying to frighten those who don't know what they're doing to behave in such a way as to minimise their risks. The insulated spanner works for those who are capable of dropping the spanner whilst disconnecting the negative terminal? But what the heck, I'm happy to withdraw the recommendation.

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