Neutral / Earth bonding

[Clifford]

On the excellent old smartgauge site (from Gibbo's days, I assume), is a guide to boat AC systems.

 

It shows various ways of combining rcd/mcb protection with shoreline and single or twin inverters, and using manual switches or relays.

 

For each combination, there are a pair of diagrams, one using an isolation transformer, one using a galvanic isolator.

 

On each IT diagram, it shows NE bonding at the output of the transformer (possibly meant to be part of the wiring of the transformer itself).

 

But on the GI diagrams there is no NE bonding at the galvanic isolator.

 

Is this meant to imply that you do not have NE bonding at the shoreline input (rely on the marina having it?, thought that was dodgy?)

 

Or is it just that NE bonding is not the topic under discussion here (I suspect the latter may be true, because no NE bonding is shown at the inverter outputs.

 

Or have I just not *got* AC at all yet? Am I a hopeless case?

 

Edited to add link, Doh!

 

http://www.smartgauge.co.uk/nb_ac_sys.html

Edited May 12, 2014 by Clifford

[nicknorman]

Shore supply already has NE bonding just like your household supply. If you NE bond on the boat, you will trip the shore bollard RCD because some of the current will now be flowing in the Earth conductor. You only need to NE bond the output of an inverter or output of a IT, both of which have "floating" L and N, not referenced to anything. (Except cheapo inverters which might have no isolation between the DC and AC side, and things like the TravelPower which are centre tapped ie there is no L and N, just "1/2L" and "1/2L").

 

Ed: I'm a bit confused by your Doh edit. Don't confuse hull bonding with NE bonding. Completely different things.

Edited May 12, 2014 by nicknorman

[Paul C]

 

But on the GI diagrams there is no NE bonding at the galvanic isolator.

 

 

 

http://www.smartgauge.co.uk/nb_ac_sys.html

 

Every diagram I can see, there is a green wire to "hull bonding point" somewhere.

 

As far as N-E bonding goes, its the responsibility of the source of the power to supply this, so its not necessary to bridge N and E on the boat when using shoreline power.

[John V]

On the GI drgs the..........Nicknorman has beaten me to it

[Clifford]

I do understand the difference between hull/earth and neutral/earth bonding. Well, I mean I know they are not the same thing, can't claim to get AC yet, it makes sense when I read it, then I have to read it again!

 

The smartgauge page DOES show earth/hull bonding.

 

Ok, thanks.

 

So the only NE bonding is on Inverter output.

 

Not wanted on shoreline input.

 

(Eureka moment! That's why combis have a relay to do NE bonding automatically (only when shoreline not connected).

 

Have I at least got that bit right?

 

Ed for typos.

Edited May 12, 2014 by Clifford

[nicknorman]

Yes, but on inverter output AND IT transformer, if you have one

[Clifford]

IT transformer, too. Yes. Because the transformer in effect breaks the shore NE bond, you have to reinstate it. Is that part of the wiring of the IT, or a user issue.

[nicknorman]

IT transformer, too. Yes. Because the transformer in effect breaks the shore NE bond, you have to reinstate it. Is that part of the wiring of the IT, or a user issue.

A user issue. The output of the GI is just a floating live and neutral, the user has to bring a boat's earth wire and a boat's neutral wire to one output terminal. Or I suppose you could add the earth wire further along, but before the RCD.

[by'eck]

I do understand the difference between hull/earth and neutral/earth bonding. Well, I mean I know they are not the same thing, can't claim to get AC yet, it makes sense when I read it, then I have to read it again!

 

The smartgauge page DOES show earth/hull bonding.

 

Ok, thanks.

 

So the only NE bonding is on Inverter output.

 

Not wanted on shoreline input.

 

(Eureka moment! That's why combis have a relay to do NE bonding automatically (only when shoreline not connected).

 

Have I at least got that bit right?

 

Ed for typos.

 

Think of the isolation transformer or the whole boat for that matter as an electrical device. They both need an earth to provide clear path to trip an RCD in the case of a failure.

 

Don't know if this helps:

 

An RCD triggers on an imbalance in the currents flowing through live and neutral legs, so will always work unless faulty. What it won't do though is provide adequate protection from a lethal shock if there are no clear conditions to trigger the imbalance.

This is why the earth/hull and neutral/earth straps are needed (on boat in case of inverter/IT derived AC or provided at substation with direct shore power). In the case of a short from live to hull an enormous current will flow only through the live leg instantly tripping the RCD.

Without the earth/hull strap, the hull could become live at 230 volts and no-one inside would know until they also touched the metal casing of single insulated connected equipment, which may be too late, or someone outside the boat touched it whilst standing on the towpath in the case of inadequately protected shore power.

Without the neutral/earth strap there are no clear conditions to trigger the imbalance in the nominal live and neutral legs should a fault occur. Although unlikely, it could be you have a lethal current presented to your body but still maintain balance in live and neutral legs so no trip.

Edited May 12, 2014 by by'eck

[Clifford]

Thank you, all. It is becoming clear, gradually!

 

I shan't be wiring any of this AC stuff myself, of course. But it is important to me to know what is going on.

[Keeping Up]

A user issue. The output of the GI is just a floating live and neutral, the user has to bring a boat's earth wire and a boat's neutral wire to one output terminal. Or I suppose you could add the earth wire further along, but before the RCD.

 

For clarification here I think you meant "The output of the IT is just a floating live and neutral ... "

[Clifford]

 

 

I did Gibbo a disservice.

 

He also has two pages on wiring an isolation transformer.

 

Output has NE bonding, of course. But there is also discussion about whether to wire the IT case to shore earth or boat earth (hull).

 

http://www.smartgauge.co.uk/iso_wire.html

[BruceinSanity]

Note that the Airlink standalone IT comes with the output side NE bonded inside the case.

[Clifford]

Ah. You'd kind of expect it to be standard. But I gather not everyone likes NE bonding.

 

Out of interest, do you know if the Airlink instructions say to earth the case ? To the shorelink earth, or the hull?

[by'eck]

Ah. You'd kind of expect it to be standard. But I gather not everyone likes NE bonding.

 

Out of interest, do you know if the Airlink instructions say to earth the case ? To the shorelink earth, or the hull?

 

How can anyone not like something that may save their life?

 

An isolation transformers input (shore power) earth should be connected to its casing/core although isolated from boat/hull. One of the IT's output legs should be connected to boat hull/earth to provide the (nominal) neutral/earth bond. In this way the shore power earth is completely isolated from boat earth to eliminate stray currents.

[Guest]

 

How can anyone not like something that may save their life?

 

An isolation transformers input (shore power) earth should be connected to its casing/core although isolated from boat/hull. One of the IT's output legs should be connected to boat hull/earth to provide the (nominal) neutral/earth bond. In this way the shore power earth is completely isolated from boat earth to eliminate stray currents.

You would wonder, but there were those on here a few years ago who claimed Gibbo (and others) were talking through their backsides when they said an N-E link should be in place. I seem to recall some even said it was dangerous to incorporate a N-E link!

 

Also the way you describe the earthing arrangements for an isolating transformer is exactly how I understand it. It seems that one manufacturer was earthing them incorrectly a few years back!

[Clifford]

Gibbo gives some reasons for earthing the case to the hull, NOT the shoreline earth, in the link I gave earlier.

 

Ed to say: But I suppose it depends on what the manufacturer has done already. If they have already wired the input earth to the case, then by earthing the case to the boat you would bypass the IT earth break! But if there is no internal Earth then you are free to choose (?).

Edited May 12, 2014 by Clifford

[Paul C]

In an ideal world, the Isolation Transformer shouldn't even be on the boat, but on the bank, so its casing is completely isolated from the boat.

[Clifford]

Yes, but then all the books would say, "provide your own, because you cant trust marinas to wire theirs up correctly". Just like we need RCD protection because we are told not to trust the bollard's.

[Paul C]

Yes, but then all the books would say, "provide your own, because you cant trust marinas to wire theirs up correctly". Just like we need RCD protection because we are told not to trust the bollard's.

 

???

 

What post was this replying to?

[Clifford]

Paul,

 

I was replying to yours. I was thinking that if an IT was off the boat completely, then it was something the mooring owner could provide. But that then we'd only be told to replicate its function ourselves anyway.

 

Was it as wind-up?

[Paul C]

Paul,

 

I was replying to yours. I was thinking that if an IT was off the boat completely, then it was something the mooring owner could provide. But that then we'd only be told to replicate its function ourselves anyway.

 

Was it as wind-up?

 

There is no motivation for the mooring owner to provide it, because the existing earthing system used with marina bollards is much cheaper. Basically, there is a marginal safety advantage, with a much greater cost, when using IT as earthing system. Some boaters are wealthy and choose to do this.

[smileypete]

Hull to earth bonding is 'equipotential bonding', if a boat is connected to the shore supply and live to hull fault occurs (say a chafed shore lead) you want the hull to remain at the same potential to the earth onshore.

 

Otherwise there could be an electrocution risk when you step ashore between boat and say landing stage, or even to nearby swimmers (not a problem here but common in parts of the USA).

 

OK most if not all marinas have shore side RCDs but there's still a remote possibility they could be faulty and fail to trip, so a second level of protection is good.

 

cheers, Pete.

~smpt~

Edited May 12, 2014 by smileypete

[BruceinSanity]

The Airlink is in a waterproof case for exterior use, just like their 230v to 110v site transformers, so earth bonding to the case doesn't arise, it's a big lump of plastic. The wiring diagram for the 230/230 boating IT is at the bottom of this page: http://www.airlinktransformers.com/boating_transformers/technical-notes/

 

There's a screen before the core earthed to the shore side so that in the event of internal failure the shoreside RCD will trip.

[nicknorman]

For clarification here I think you meant "The output of the IT is just a floating live and neutral ... "

Oops! you're right of course.