Alternator Earth

[simon&jan]

Our boat is not hull bonded from wot i can see ,all the earth leads seem to go to the starter battery neg for some reason ,is this something we should worry about ???

[MoominPapa]

 

No, with a 'traditional' fully isolated marine installation, the theory is that there is no electrical connection to the hull at all.

 

We're descending into buts and maybes here. The conversation started with a system where the starter is earthed to the engine, and the path from the engine block to the battery negative is provided by a second starter solenoid. There's likely to be a path from the engine to the steel hull, and if there's a leakage path from the hull to the battery negative (eg the radio aerial braid) then that provides an alternative path from starter case to battery if the solenoid fails. Unless the system has earth-leakage detection, you wouldn't know about the alternative paths until the solenoid failed, when they could suddenly hot. Of course in the system where there's a big fat cable from the engine block to the battery the same applies to failure of that cable, but a solenoid must be orders of magnitude more likely to fail.

 

MP.

[FORTUNATA]

My air cooled Lister has no heater plugs but, however the case may be, I can only suggest any isolated DC circuit should be tested with a multimeter to make sure it is isolated. That means starter motor terminal and alternator plus negative battery terminal should be totally ungrounded if possible. IN such a case, there needs to be double pole circuit protection in the DC system, not just on the positive pole as in grounded DC systems. And if you really want to be flashy there's a system called "discrimination" that supposedly offers sure fire tripping protection to make the system safer (although it's said nobody uses it)
I once did a test on my boat given it was wired by an AC electrician years ago and I got curious about the AC and DC. I found that if I rotated my battery selector switch to Off, as would be expected all the DC was dead. No horn, stereo, water pump zilch. Then I plugged in the inverter charger. Lo and behold, I now had all DC systems working again, even though my isolater was Off. Just as I expected, I suppose. What happened was the DC charge current from the inverter charger was automatically activated and thereby supplying all DC circuits, powered by an AC mains external hook-up supply. I didn't allow this to happen for any length of time but it was just a test. I recall the AC service guy told me on phone he had bypassed the selector switch normal function and simply wired up two 12 volt batteries in parallel via the said rotary switch.
If I ever find the time I will one day change this and use diode isolator, battery sensing for regulator and a totally floating DC system with AC grounded as usual.
Even when done properly I heard cases of swimmers being zapped around boats if fault current grounds to the hull ground.

 

The same system could be applied to heater plugs as Listers and probably others used to apply to starter motors - they would be 'earthed' via an extra starter solenoid, only for the duration of the starting process.

 

Tim

[by'eck]

This subject has been covered before but to recap here are some of the reasons all battery DC negatives should be common and strapped to boat hull at a single point.

 

Reasons for single ground point to hull for DC negative. Note this doesn't in any way remove the need for a dedicated DC negative cable feed to all DC devices.

1) Safety - any short from live AC to battery negative will go to hull/ground & trip RCD

2) Many engine ancillaries such as starter motor, alternator or senders rely on engine block being at DC negative. Even if they have a dedicated negative terminal its often connected internally to body of ancillary. Since block is bolted to hull it is also at DC negative anyway.

3) Following on from 2), you know there will be no stray paths from DC negative to hull which may otherwise be through a thin cable or sensitive electronics.

4) Provides a common ground for stray currents induced into cable runs which may otherwise cause RF interference or hum on audio systems.

5) A short from DC positive to hull without a ground strap will cause a current flow through any of possibly many stray paths. This may not blow the fuse or trip the breaker but could damage any item creating that stray path in a greater or lesser time.

6) Also following on from 2), many car electronics have their mounting chassis at DC negative, so a connection or short to hull cannot easily be avoided with such, although the main negative power feed would be via a dedicated cable.

So to sum up, in an ideal situation the DC negative hull connection will never carry any current, but in the event of a fault or unforeseen situation, will provide a single safe path to ground, quickly tripping the circuit protection in the case of a DC positive short to hull. You will also be turning a tenuous situation into a known one and so create clear conditions for safety trips and fuses to work quickly and effectively. Stray current electrolytic errosion on the hull is not an issue as long as its DC negative that's grounded.

Note that an AC ground point to hull is also required for the RCD safety device(s) to be effective. This should be close but separate to avoid the two straps becoming detached but remaining together and thus allowing the admittedly distant possibility of the DC system becoming live to AC mains.

[FORTUNATA]

"2) Many engine ancillaries such as starter motor, alternator or senders rely on engine block being at DC negative. Even if they have a dedicated negative terminal its often connected internally to body of ancillary. Since block is bolted to hull it is also at DC negative anyway."
It struck me that perhaps isolated DC systems have in mind a certain alternator which might explain why the O.P. was led to believe his alternator wasn't intended to be fitted as normal. The standard test for an alternator which has a negative regulator excitation voltage is positive meter lead to B+ and negative probe to the Field connection. Normally it would be between B+ and the case (scratched). If I'm not mistaken that would mean that such an alternator would not be grounded to the engine block via its case as happens when a positive excitation control alternator is installed. Starter motor is not connected to alternator in such a case.
I tried a brief look on marine forums to see if this was the case but really didn't notice a great deal of debate on isolated earth.
I assume there have been many cases of people being severely shocked when assuming the DC system is safe and this led to the idea to totally isolate D.C. However, it should be stressed that the accidents tend to have taken place when AC ground is led almost direct to the battery negative and then the hull ground failed.

 

 

 

 

This subject has been covered before but to recap here are some of the reasons all battery DC negatives should be common and strapped to boat hull at a single point.

 

Reasons for single ground point to hull for DC negative. Note this doesn't in any way remove the need for a dedicated DC negative cable feed to all DC devices.

1) Safety - any short from live AC to battery negative will go to hull/ground & trip RCD

2) Many engine ancillaries such as starter motor, alternator or senders rely on engine block being at DC negative. Even if they have a dedicated negative terminal its often connected internally to body of ancillary. Since block is bolted to hull it is also at DC negative anyway.

3) Following on from 2), you know there will be no stray paths from DC negative to hull which may otherwise be through a thin cable or sensitive electronics.

4) Provides a common ground for stray currents induced into cable runs which may otherwise cause RF interference or hum on audio systems.

5) A short from DC positive to hull without a ground strap will cause a current flow through any of possibly many stray paths. This may not blow the fuse or trip the breaker but could damage any item creating that stray path in a greater or lesser time.

6) Also following on from 2), many car electronics have their mounting chassis at DC negative, so a connection or short to hull cannot easily be avoided with such, although the main negative power feed would be via a dedicated cable.

So to sum up, in an ideal situation the DC negative hull connection will never carry any current, but in the event of a fault or unforeseen situation, will provide a single safe path to ground, quickly tripping the circuit protection in the case of a DC positive short to hull. You will also be turning a tenuous situation into a known one and so create clear conditions for safety trips and fuses to work quickly and effectively. Stray current electrolytic errosion on the hull is not an issue as long as its DC negative that's grounded.

Note that an AC ground point to hull is also required for the RCD safety device(s) to be effective. This should be close but separate to avoid the two straps becoming detached but remaining together and thus allowing the admittedly distant possibility of the DC system becoming live to AC mains.

 

[by'eck]

"2) Many engine ancillaries such as starter motor, alternator or senders rely on engine block being at DC negative. Even if they have a dedicated negative terminal its often connected internally to body of ancillary. Since block is bolted to hull it is also at DC negative anyway."

It struck me that perhaps isolated DC systems have in mind a certain alternator which might explain why the O.P. was led to believe his alternator wasn't intended to be fitted as normal. The standard test for an alternator which has a negative regulator excitation voltage is positive meter lead to B+ and negative probe to the Field connection. Normally it would be between B+ and the case (scratched). If I'm not mistaken that would mean that such an alternator would not be grounded to the engine block via its case as happens when a positive excitation control alternator is installed. Starter motor is not connected to alternator in such a case.

I tried a brief look on marine forums to see if this was the case but really didn't notice a great deal of debate on isolated earth.

I assume there have been many cases of people being severely shocked when assuming the DC system is safe and this led to the idea to totally isolate D.C. However, it should be stressed that the accidents tend to have taken place when AC ground is led almost direct to the battery negative and then the hull ground failed.

 

 

 

 

90% of European alternators have negative field control (i.e. the regulator operates between battery negative/ground and one of the field brushes). I don't see why field control has a bearing on whether there is a dedicated DC negative (B-) output terminal (isolated return) or whether its via the casing (or both).

 

DC and AC hull connection studs should be separate but close together, this to avoid the admittedly unlikely event of a shared connection parting from the hull leaving an ungrounded connection between the two circuits, and so compromising protection devices.

Edited April 15, 2014 by by'eck

[Sir Nibble]

We're descending into buts and maybes here. The conversation started with a system where the starter is earthed to the engine, and the path from the engine block to the battery negative is provided by a second starter solenoid. There's likely to be a path from the engine to the steel hull, and if there's a leakage path from the hull to the battery negative (eg the radio aerial braid) then that provides an alternative path from starter case to battery if the solenoid fails. Unless the system has earth-leakage detection, you wouldn't know about the alternative paths until the solenoid failed, when they could suddenly hot. Of course in the system where there's a big fat cable from the engine block to the battery the same applies to failure of that cable, but a solenoid must be orders of magnitude more likely to fail.

 

MP.

In practice when this fault occurs as a rule the starter solenoid fails to engage.

[FORTUNATA]

 

90% of European alternators have negative field control (i.e. the regulator operates between battery negative/ground and one of the field brushes). I don't see why field control has a bearing on whether there is a dedicated DC negative (B-) output terminal (isolated return) or whether its via the casing (or both).

 

DC and AC hull connection studs should be separate but close together, this to avoid the admittedly unlikely event of a shared connection parting from the hull leaving an ungrounded connection between the two circuits, and so compromising protection devices.

Much boils down to preference. The system you described seems to be very popular with American yachts and, at the end of the day, so long as the system is correctly and safely installed all is well. I did notice that John C Payne devoted a few paragraphs plus a schematic to the isolated DC system and states his clear preference for that system but he also stresses the reasons why you need double pole circuit protection for the latter system.

I mentioned the field control because clearly the alternator case does need to be electrically isolated from the engine block and from there to the hull ground. If I planned on an isolated system I'd naturally be testing the alternator with a meter to make sure it is isolated (otherwise the system is rendered redundant).

Anyway, I know this hasn't caught on at all inland and am led to believe most vessels go with the usual DC engine ground.

[Yarwell]

Hi

Hope someone can give me some advice on nan alternator that seems to be draining batteries.

I had seven new sealed 110amp domestic batteries fitted early last year. Everything was fine and the batteries were performing well showing 13.12 amps after a days cruising (6 hrs). I then had an alternator problem ( it had died) and had it changed by Penine Cruisers at Skipton. Everything seemed once again to be OK. When I left the boat over the Winter I turned off all the isolators but when I visited the boat for a bit of routine maintenance I found that the batteries had been totally drained. I teased the alternators and managed to get them recharged over four days using shore power through an inverter. When I came back to the boat 1month later again I found that the batteries had been totally discharged. I took the batteries back to the supplier for a deep charge. When I refitted them I left the alternator strap off as this was not routed through the isolator and I thought that this might be the problem. I have just returned and am pleased to say that the batteries have held their charge. This means that the alternator must be causing the leak to earth. My question is has the alternator been miss wired somehow or could the fault be within the actual alternator ( somebody has suggested a faulty diode ((is this easily sorted?))

 

Many thanks

 

Yar Well ( I wish I knew about these things)

[Tony Brooks]

I doubt you mean 13.12 amps, more likely 13.12 volts and if so that is far too low after 6 hours cruising unless you have had a very heavy load on all the time or have a faulty battery (shorted cell). If neither are the case then yes, your alternator probably has fault and did so from the start.

 

The BSS requires alternator charging leads\to be routed via the master switch despite Gibbo's protetsts so although your way is less likely to cause problems it should be a BSS fail.

[Paul C]

 

The BSS requires alternator charging leads\to be routed via the master switch despite Gibbo's protetsts so although your way is less likely to cause problems it should be a BSS fail.

 

Are you sure?

[Yarwell]

Hi

Yes your right 13.12 volts. Re the routing through master switch/isolator, I have just had a BSS and the strap from the alternator was directly onto the batteries and it passed OK. As I said the batteries have been checked and deep charged so no apparent cell problems. How do I check if it is the diode in the Alternator and is it easy to replace?

 

Yar Well ( as I said I know nothing )

[ditchcrawler]

 

Are you sure?

I am unless its fused.

I isolate everything except the bilge pump and solar panel when I leave the boat.

[Tony Brooks]

Hi

Yes your right 13.12 volts. Re the routing through master switch/isolator, I have just had a BSS and the strap from the alternator was directly onto the batteries and it passed OK. As I said the batteries have been checked and deep charged so no apparent cell problems. How do I check if it is the diode in the Alternator and is it easy to replace?

 

Yar Well ( as I said I know nothing )

 

Unless you strip the alternator you are unlikely to positively identify an open circuit diode or dry joint. However as a modern alternator charging well charged batteries is probably going to regulate at 14.4+ I think that there is every chance the alternator is faulty so worth taking off for testing at your local autoelectrical company. . HOWEVER if the alternator is supplying a significant current it will pull the voltage down but after 6 hours running I find 13.12 rather lower than I would expect under almost any load.