do I need to earth the AC?

[Wittenham]

I have the set up below; the plan is to have the generator recharge batteries.  It looks like I am about to get a sunny spot on Agenda 21, so will add solar in the near-ish future.  Most of the boat, which does not have an engine, will run off DC, but the inverter will supply AC as needed.  The fridge will be one of the 'needed'.  I have a Victron Multiplus and have earthed it to the hull.  Do I also need to earth the AC separately?  

 

 

thanks in advance,

 

greg

[WotEver]

20 minutes ago, Wittenham said:

I have a Victron Multiplus and have earthed it to the hull.  Do I also need to earth the AC separately?  

No, because the earth from the Multi casing is the same thing.

[Wittenham]

Thank you for that.  To be sure I understand, the bible of Smartgauge says:

AC system and DC system

If both electrical systems are installed then all of the above applies. i.e. it is imperative that both systems be bonded to the hull.

There is also another scenario in this case to further convince the doubters.

Assume the AC system is bonded to the hull but the DC system is not.

Some equipment is connected to both systems. This sounds rare - in actual fact it isn't, dual voltage fridges, battery chargers and inverters are all connected to both systems.

A fault in one of these items could cause AC mains to be presented to the DC side. If both systems are bonded to the hull, this will instantly cause the incoming fuses or circuit breakers to blow.

If one of the systems is isolated from the hull this will not happen. The result will be that the DC system (which we all assume is safe to touch, and which usually has components with insulation rated for about 50 volts) will be sat at 230 volts with respect to the hull or the other electrical system. Clearly this is highly dangerous.

In summary, whatever electrical system is fitted, it is imperative that the system is bonded to the hull.

 

Is the logic that because the AC and DC meet at the Multiplus, the single earth to the hull coming from that box is sufficient?

 

Does the answer change if I connect the Multiplus to shore power?

 

[Stephen Jeavons]

Assuming the shore power connects to the same point as your generator i.e. on the south side of the galvanic isolator, then it's the same thing. It shouldn't make any difference as you're not using the shore earth on the boat. My system is exactly like this except that I have an isolation transformer instead of the galvanic isolator

Edited May 20, 2019 by Stephen Jeavons
correct spelling

[WotEver]

3 hours ago, Wittenham said:

Is the logic that because the AC and DC meet at the Multiplus, the single earth to the hull coming from that box is sufficient?

No. 

 

It is because the case of the Multiplus is connected to the 230V earth output. 

 

My preferred way of wiring the earth to hull connection is to run a 4mm² cable from the consumer unit earth busbar to the hull bonding point adjacent to but not concurrent with the main D.C. -ve bond. You could do this too if you wish but the case bond achieves the same thing. 

3 hours ago, Wittenham said:

Does the answer change if I connect the Multiplus to shore power?

Nope

[Guest]

17 minutes ago, WotEver said:

No. 

 

It is because the case of the Multiplus is connected to the 230V earth output. 

 

My preferred way of wiring the earth to hull connection is to run a 4mm² cable from the consumer unit earth busbar to the hull bonding point adjacent to but not concurrent with the main D.C. -ve bond. You could do this too if you wish but the case bond achieves the same thing. 

Nope

Same here, that is how I did the job.

[Stephen Jeavons]

Incidentally, is there a reason that it is recommended to bond the DC -ve to a different earthing post on the hull but adjacent to the AC earthing post? I bonded everything to the same earthing post.

[Guest]

8 minutes ago, Stephen Jeavons said:

Incidentally, is there a reason that it is recommended to bond the DC -ve to a different earthing post on the hull but adjacent to the AC earthing post? I bonded everything to the same earthing post.

The explanation I was given: Should the nut come loose, in the event of a 240V short to earth, it could find its way back up the 12V earth and put 12V stuff at 240V potential. 

Like you I had a single stud to begin with, but changed it to two.

Edited May 20, 2019 by Guest

[Wittenham]

thanks all, very useful.  For belt and braces, I will do this, as well:

 

run a 4mm² cable from the consumer unit earth busbar to the hull bonding point adjacent to but not concurrent with the main D.C. -ve bond. 

[Stephen Jeavons]

15 minutes ago, catweasel said:

The explanation I was given: Should the nut come loose, in the event of a 240V short to earth, it could find its way back up the 12V earth and put 12V stuff at 240V potential. 

Like you I had a single stud to begin with, but changed it to two.

Got it!  I have a nyloc stainless nut along with a spring washer on top of all my tags so unlikely to come loose. I understand the principal though

Many thanks for the explanation.

[cuthound]

1 hour ago, catweasel said:

The explanation I was given: Should the nut come loose, in the event of a 240V short to earth, it could find its way back up the 12V earth and put 12V stuff at 240V potential. 

Like you I had a single stud to begin with, but changed it to two.

 

This ^^^^ 

[WotEver]

2 hours ago, Stephen Jeavons said:

Got it!  I have a nyloc stainless nut along with a spring washer on top of all my tags so unlikely to come loose. I understand the principal though

Many thanks for the explanation.

Yes but (vanishingly unlikely) the stud could come adrift from the hull. Your nylock nut will keep everything together but no longer connected to the hull. Has it ever happened? Probably not, but best practise and all that...

[Stephen Jeavons]

13 hours ago, WotEver said:

Yes but (vanishingly unlikely) the stud could come adrift from the hull. Your nylock nut will keep everything together but no longer connected to the hull. Has it ever happened? Probably not, but best practise and all that...

T'aint a stud. it's a stainless bolt through a hole drilled in a rib of the boat followed by the wiring lugs, a spring washer and a nyloc nut. That lot isn't going to come loose. However, just to stick with convention I may separate AC and DC by drilling a second hole