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A few questions re connecting shore power


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Our boat has never been fitted with a permanent shore power connection, but because our "temporary" use of the land line is becoming more regular, i think its about time we got hard wired so to speak. I have a few questions.

 

1) As far as I can tell our boat doesn't seem to have a DC ground, though I'm a long way from it at the moment so I can't check for certain. I know when installing AC power it's essential that AC earth and DC negative are bonded to the hull so I assume it's a case of taking a lead from the battery negative terminal and bolting it to a floor bearer, for example.

 

2) Ideally I would have liked the consumer unit at the bow, primarily because any AC power appliances will be up front, but I understand the DC and AC ground should be at the same "point", so I guess the unit has to go astern to achieve this?

 

3) We always moor at the marina bow - to, so a power lead has to go all the way to the stern assuming that's where the consumer unit will be. I notice other boats on the marina have a shore power plug at the bow, which presumably means they have another lead within the boat heading internally back astern. (again, presuming the consumer unit is at the stern) Would that be better than having an external lead trailing along the pontoon or the cabin roof?

 

4) Is an external power lead plug mounted on the bulkhead preferable to having the lead routed say through a door/window or other opening so the plug can be mounted internally? (We don't have any cratch/cockpit covers by the way.)

Edited by Neil2
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Our boat has never been fitted with a permanent shore power connection, but because our "temporary" use of the land line is becoming more regular, i think its about time we got hard wired so to speak. I have a few questions.

 

1) As far as I can tell our boat doesn't seem to have a DC ground, though I'm a long way from it at the moment so I can't check for certain. I know when installing AC power it's essential that AC earth and DC negative are bonded to the hull so I assume it's a case of taking a lead from the battery negative terminal and bolting it to a floor bearer, for example.

 

2) Ideally I would have liked the consumer unit at the bow, primarily because any AC power appliances will be up front, but I understand the DC and AC ground should be at the same "point", so I guess the unit has to go astern to achieve this?

 

3) We always moor at the marina bow - to, so a power lead has to go all the way to the stern assuming that's where the consumer unit will be. I notice other boats on the marina have a shore power plug at the bow, which presumably means they have another lead within the boat heading internally back astern. (again, presuming the consumer unit is at the stern) Would that be better than having an external lead trailing along the pontoon or the cabin roof?

 

4) Is an external power lead plug mounted on the bulkhead preferable to having the lead routed say through a door/window or other opening so the plug can be mounted internally? (We don't have any cratch/cockpit covers by the way.)

1/ Your boat may actually have a DC ground bearing in mind the engine is typically well connected to DC -ve. Most likely however it will have a rather poor ground to hull via exhaust pipe, fuel pipe, control cables, prop shaft etc. So in order to avoid multiple ground points and thus possible hull currents, I'd take a lead from the engine to the hull.

 

2/ you could run the cable inside the boat from the front to a rear consumer unit. However, whilst it is recommended to have the DC and AC ground points adjacent, I cant think of a strong reason for that so perhaps you can just have the CU and AC ground at the front.

 

3/ see 2. However you could also consider having shore connectors front and rear - you may not always be at the same mooring! If you have 2 inlets you must have a switching system so that the socket not in use isn't live.

 

4/ I'd certainly want an external connector. You don't want to be forced to have a window open in cold weather nor to have unsightly cables dangling inside the boat.

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Nick, re point 2. Should you not have the CU close to the mains inlet? And would there be any problem with running a long earth wire from that CU back to the engine area?

For the BSS there is no requirement to have a CU near the inlet. If you want to comply with ISO13297 then yes, there should be an overcurrent breaker within 0.5m (or 3m if trunked) but that doesn't have to be the main CU.

 

I'm still not sure why the hull Earth bond needs to be near the DC one. But I suppose you could run a long Earth wire, just not sure why you'd want to bother!

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For the BSS there is no requirement to have a CU near the inlet. If you want to comply with ISO13297 then yes, there should be an overcurrent breaker within 0.5m (or 3m if trunked) but that doesn't have to be the main CU.

 

I'm still not sure why the hull Earth bond needs to be near the DC one. But I suppose you could run a long Earth wire, just not sure why you'd want to bother!

 

The Smartguage site advises that

 

All bonding should be done at one central point. It is not acceptable to bond various parts of the system in various separate places. This can cause voltage differentials between various parts of the hull which can lead to stray current erosion

 

I don't doubt this is correct but i'd like to understand it better.

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For the BSS there is no requirement to have a CU near the inlet. If you want to comply with ISO13297 then yes, there should be an overcurrent breaker within 0.5m (or 3m if trunked) but that doesn't have to be the main CU.

 

I'm still not sure why the hull Earth bond needs to be near the DC one. But I suppose you could run a long Earth wire, just not sure why you'd want to bother!

 

 

 

The Smartguage site advises that

 

All bonding should be done at one central point. It is not acceptable to bond various parts of the system in various separate places. This can cause voltage differentials between various parts of the hull which can lead to stray current erosion

 

I don't doubt this is correct but i'd like to understand it better.

 

Correct saved me saying it

Edited by Geo
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I also go along with Nick in that I can't see how a single AC earth bond well away from the DC bond can cause stray currents. I Know at least one "quality" builder on one boat ash the AC bond maybe a metre or more away from the DC bond.

 

A definitive explanation from someone who really knows would be good.

 

I can see that having both bonds close together makes tracing and fault finding the system easier.

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The Smartguage site advises that

 

All bonding should be done at one central point. It is not acceptable to bond various parts of the system in various separate places. This can cause voltage differentials between various parts of the hull which can lead to stray current erosion

 

I don't doubt this is correct but i'd like to understand it better.

 

I know it does. However it doesn't say exactly what mechanisms could exist that cause current to flow from the DC system to the AC system. Surely that shouldn't be happening!

Correct saved me saying it

So can you explain how there can be a circuit between the mains AC Earth and the DC negative? I'll remind you that for current to flow, you need to have a complete circuit and a source of voltage, not just two things connected together.

And let's remember that most boats have multiple DC -ve to hull connections via the various bits of hardware that are connected to both the engine and the hull.

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The Smartguage site advises that

 

All bonding should be done at one central point. It is not acceptable to bond various parts of the system in various separate places. This can cause voltage differentials between various parts of the hull which can lead to stray current erosion

 

I don't doubt this is correct but i'd like to understand it better.

Not sure if the regs have changed, but when I fitted out my boat the requirement was that AC and DC must NOT share an earth point (bolt), but should be earthed separately AND close to each other.

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3/ see 2. However you could also consider having shore connectors front and rear - you may not always be at the same mooring! If you have 2 inlets you must have a switching system so that the socket not in use isn't live.

 

what is wrong with having a live socket at one end of the boat?

it could be useful for operating a portable appliance when needed - drill, lamp, vacuum, etc.

 

 

.............. OH! you meant a live male plug. frusty.gif

 

a common mistake, either in description or, god forbid in execution (which also often leads to death). rolleyes.gif

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Not sure if the regs have changed, but when I fitted out my boat the requirement was that AC and DC must NOT share an earth point (bolt), but should be earthed separately AND close to each other.

Likewise. "within 75mm" I seem to recall. Not sure how big the risk is if they are a long way apart, but I followed the advice of the day (plus I wanted to pass my boat inspection.)

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Let's take an example of a power inlet near the bow, with the CPC locally bonded to the hull, and a power-assist inverter near the stern.

 

It is most likely that along its path from bow to stern that the CPC (earth) wire will acquire an induced voltage (albeit of very high impedance). The inverter will be locally bonded to the hull at the stern with the result that the CPC is now bonded at two points with a PD between those two points.

 

A perfect scenario for stray current corrosion.

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Let's take an example of a power inlet near the bow, with the CPC locally bonded to the hull, and a power-assist inverter near the stern.

 

It is most likely that along its path from bow to stern that the CPC (earth) wire will acquire an induced voltage (albeit of very high impedance). The inverter will be locally bonded to the hull at the stern with the result that the CPC is now bonded at two points with a PD between those two points.

 

A perfect scenario for stray current corrosion.

I recall that the guy who does our boat safety claimed to have got a "tingle" on on boat owing to a similar scenario. I have forgotten the exact details, as it was some years ago.

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I recall that the guy who does our boat safety claimed to have got a "tingle" on on boat owing to a similar scenario. I have forgotten the exact details, as it was some years ago.

I have read 120V on a CPC before now but only when the end was floating and only with a high impedance DMM. Yes, enough to feel and certainly enough for electrolytic corrosion.
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what is wrong with having a live socket at one end of the boat?

it could be useful for operating a portable appliance when needed - drill, lamp, vacuum, etc.

 

 

.............. OH! you meant a live male plug. :banghead:

 

a common mistake, either in description or, god forbid in execution (which also often leads to death). :rolleyes:

Ok fair cop! Although I'll just mention that you can have male sockets and female plugs too. It's all very confusing!

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Ok fair cop! Although I'll just mention that you can have male sockets and female plugs too. It's all very confusing!

With XLR connectors we used to refer to a cable mounted female plug in addition to male sockets.
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... what mechanisms could exist that cause current to flow from the DC system to the AC system. Surely that shouldn't be happening! ...(snipped)

 

 

 

Your nice switch mode charger will be connected to the battery and the mains and any switch mode electrical noise will be dumped down the mains earth also connected to the battery and the hull by the negative earth bond.

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Let's take an example of a power inlet near the bow, with the CPC locally bonded to the hull, and a power-assist inverter near the stern.

 

It is most likely that along its path from bow to stern that the CPC (earth) wire will acquire an induced voltage (albeit of very high impedance). The inverter will be locally bonded to the hull at the stern with the result that the CPC is now bonded at two points with a PD between those two points.

 

A perfect scenario for stray current corrosion.

 

OK I think I understand that now, it's not the separation of the grounding points per se that causes the problem, it's the practical effect that plugging a (bonded) battery charger creates - right?

 

However, and I suspect I'm going to look stupid asking this question, if the battery charger is plugged into an earthed consumer unit, isn't it already earthed? Why does the charger have to be separately bonded to the hull?

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OK I think I understand that now, it's not the separation of the grounding points per se that causes the problem, it's the practical effect that plugging a (bonded) battery charger creates - right?

 

However, and I suspect I'm going to look stupid asking this question, if the battery charger is plugged into an earthed consumer unit, isn't it already earthed? Why does the charger have to be separately bonded to the hull?

Not a stupid question at all. If the CU is aft and earthed locally and the incoming is at the bow and earthed locally then there's the potential (sorry) for corrosion.

 

Most high-end inverters insist that you Earth the case too...

 

Not so much with chargers, but some do.

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I would like to see that regulation

Pretty certain it was in the old red book when I fitted our boat out. Not looked at the new BSC that much so not sure if it states similar in there or not. The more I think about it the more I am convinced it was "within 75mm but the ac and dc bonds couldn't be on the same stud (or words to that effect.)

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I would like to see that regulation

ISO 13297: 2000 version stated "4.2 The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as practicable to the battery (d.c.) negative terminal."

 

But in the 2014 version of the standard the requirement to be close to the craft's DC negative ground has gone.

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