Neutral-Ground-TVS zener diode

[Locquatious]

We are replacing our existing 6kW generator in kinda-like. The old installation did not bond boat ground with the neutral at the generator. We want to tie neutral to boat ground at the generator yet avoid electrolysis. We want to use a TVS zener diode in the ground circuit.

Dissimilar metal galvanic voltage, based upon the electrolytic series should never exceed 1.4V. So diode(s) with at least 1.4V forward voltage should block dissimilar metal electrolysis.

The generator has a 30A circuit breaker on it. I have read that any diode or other isolation device should be rated 135% above or 40.5A.

A TVS zener diode 5KP5.0CALFCT-ND‎ from Digikey starts conducting above 5V and is in full conductance above 9.2V. It blocks and conducts in both directions equally. It easily will ground high current pulses and can withstand 100 half cycles at 60A 60 Hz. That is the TVS should be able to pass any generator current to ground till the circuit breaker kicks.

The 5-9V blockage should stop electrolysis. <9V AC should be relatively safe for people. The TVS could be crimped into the ground wire circuit. It costs $3.10.

Any thoughts or concerns on the approach?

[BillKny]

It's always difficult discussing "grounds" because the terminology is a bit murky and people do not understand the different kinds of grounds and how they are used and they use terms that are either meaningless, or just wrong.

You say the "boat ground" was "not connected to neutral at the generator". When you say "boat ground" I assume you are talking about the AC safety ground, aka the "safety earth", or the "green wire." If so, this is a dramatic failure of the old installation to follow standard practice, basically disabling the entire safety ground system when the generator is running.

A couple rules about the "Safety Ground". It must be connected to neutral ONLY at the source of power and NOWHERE else. In other words when you are on generator power the Safety Ground must be connected to Neutral at the generator and nowhere else.

And this next part is important... When you are on shore power, the Safety Ground must ONLY be connected to neutral at the marina's electric distribution box on shore, which is now the source of power and NOWHERE on the boat. Typically, when the generator is turned off, and the boat uses shore power there is a double-pole/double-throw transfer switch that breaks both neutral and hot wires from the generator to the AC system. This breaks the Safety Ground connection to the ship's neutral wire.

If you follow the guidelines, the connection between the Safety Ground and the generator Neutral is never a concern while connected to shore power, because that connection must not exist. Your concern about corrosion and stray currents shouldn't be an issue when you are running generator power without a shore power connection

You seem to want to build a galvanic isolator from scratch to protect yourself from stray current corrosion when connected to shore power(why?). You should buy one that complies with ABYC guidelines for fault tolerance and robustness.

[Frankly]

I think there is a little stray current corrosion and galvanic corrosion conflation going on here. The stray current is normally an issue when on shore power and can be mitigated with an isolation transformer or galvanic isolator. Galvanic corrosion is taken care of by a proper bonding system and zinc/ aluminum anodes. End effect is similar (dissappearing submerged metals but causes differ). In the old days of the zinc savers ( connected between the AC ground and ships ground/ engine/ 12V negative) they were made from a couple of diode arrays (don't recall bridge or full wave). When the "powers that be" changed the location to connected in series with the green shore power lead some fail safe requirements got included. It was never clear exactly how you do that in lightning country.

You have to kind of take it or leave it in most shore power situations but for your own craft clean up the power system and then bonding/ connecting the generator ground and neutral should not present a stray current situation. RCD breakers and GFI receptacles go a long way toward keeping every thing proper/ safe shipboard. Use the SP/Zener diodes in your lightning mitigation system, but need to raise the clamp voltage a little.


Frankly

[Locquatious]

The new 6kW generator is mounted in the boat and is a separate circuit from any marina connections. The marina connections were not a part of this thread.

The old generator did not have a neutral bonded to ground. The ground wire, green if you like was not connected. I want to connect it to a ground plate on the boat.

Galvanic corrosion will occur whenever there are dissimilar metals. The copper ground plate is a different metal than those on the generator. There is always a chance of corrosion at the generator neutral/ground post, the wires and/or connectors, and the attachment to the ground plate.

I have used high current diodes in the past to prevent ground wire corrosion on electric water heaters. So far they have worked well.

The question is, will the TVS diode perform the same function as the other high cost pieces of equipment? Too many times we open up expensive equipment to find one $3.10 TVS diode inside.

[smac999]

It sounds like you are trying to build a galvanic isolator on a gen? It is not needed and will do nothing.

What is needed is to make sure both the hot and neutreal get switched on the shore / gen transfer switch.

If the neutral is not switched you will now end up with a nuetral to ground bond on the boat while plugged into shore power. Making the shore power galvanic isolator bypassed.

[Frankly]

"You can lead a horse to water, but you can not make him drink" , some old wise man once said.


Frankly

[smac999]

Originally Posted by Locquatious

Galvanic corrosion will occur whenever there are dissimilar metals. The copper ground plate is a different metal than those on the generator. There is always a chance of corrosion at the generator neutral/ground post, the wires and/or connectors, and the attachment to the ground plate.
.

No it won’t. Galvanic corrosion only happens when both objects are underwater. The circuit is a compleate loop with current flowing through the water and returning on the wire. Creating a full circuit loop. 2 things inside the boat joined together is not a loop or circuit.

[Locquatious]

This from the Northern Lights Installation Manual:

"There is no consensus of opinion on whether the neutral conductor should be connected to the bonding system (grounded) or not (floating ground). Grounding the neutral may increase electrolytic corrosion. Not grounding the neutral creates a potential shock hazard. The American Boat and Yacht Council recommends grounding the neutral at the generator for safety reasons, though this may shorten the life of heat exchangers and
other components. Northern Lights heartily recommends grounding the neutral since personal safety takes priority over all other considerations."

It would seem that galvanic corrosion related to bonding neutral to ground on a generator is a concern. We are trying to eliminate the galvanic corrosion while maintaining the ground safety.

What does our TVS diode solution lack over the more expensive boxes?

[YANDINA]

Originally Posted by Locquatious

We are replacing our existing 6kW generator in kinda-like. The old installation did not bond boat ground with the neutral at the generator. We want to tie neutral to boat ground at the generator yet avoid electrolysis. We want to use a TVS zener diode in the ground circuit.

Dissimilar metal galvanic voltage, based upon the electrolytic series should never exceed 1.4V. So diode(s) with at least 1.4V forward voltage should block dissimilar metal electrolysis.

The generator has a 30A circuit breaker on it. I have read that any diode or other isolation device should be rated 135% above or 40.5A.

A TVS zener diode 5KP5.0CALFCT-ND‎ from Digikey starts conducting above 5V and is in full conductance above 9.2V. It blocks and conducts in both directions equally. It easily will ground high current pulses and can withstand 100 half cycles at 60A 60 Hz. That is the TVS should be able to pass any generator current to ground till the circuit breaker kicks.

The 5-9V blockage should stop electrolysis. <9V AC should be relatively safe for people. The TVS could be crimped into the ground wire circuit. It costs $3.10.

Any thoughts or concerns on the approach?

ABYC specifications for galvanic isolators require them to be able to carry over 135% of the continuous AC rating.
A 40 amp TVS diode for $3.10 can carry 40 amps for 10 MICROSECONDS, NOT CONTINUOUSLY. It would be useless for providing the ground connection with isolation.
You can get 1.2 volt galvanic isolators for under $100 that are rated for circuits up to 50 amps AC.
Yandina is the only manufacturer of a 2.5 volt, 50 amp isolator which would be a better solution.

[Locquatious]

Yandina
Great answer. Thanks.
ABYC I think requires 1 hour for <50A. Did not see continuously.

[BillKny]

Originally Posted by smac999

No it won’t. Galvanic corrosion only happens when both objects are underwater. The circuit is a compleate loop with current flowing through the water and returning on the wire. Creating a full circuit loop. 2 things inside the boat joined together is not a loop or circuit.

^^^^ Like he said! You are running down a solution to a non-problem. For there to be stray current corrosion there has to be current, which means a CIRCUIT, which means a closed path from DC positive to DC negative.

But if you really want to build a galvanic isolator, the ABYC requires the following:

• A current raitng at least equal to the rating of the AC circuit on the boat. (in your case the generator's peak capacity)
  
• Able to dissipate the heat generated by a short that is 135% of the rated capacity of the isolator without the external temperature exceeding 90C
  
• The ability to withstand a short of 100X the amperage rating of the isolator for at least one AC cycle.
  
• The ability to conduct up to 3 amps of AC current while still blocking DC current.
  
• A status monitoring system that can alarm on any of the following:
  • A failure to conduct above 2.5 V DC
  • A failure to open below 1.0V DC
  • A failure in the shorted or open position
  • A loss of continuity on the grounding circuit for any reason
  • The diodes are biased into conduction
  • The status monitor circuit itself fails

That is what the more expensive ones do...

In your proposed design, if your diodes were to fail, leaving you with no ground circuit how would you ever know?

[Locquatious]

BillKy

Thanks. I am beginning to appreciate the effort that goes into a proper galvanic isolator. They are more than a diode in a box.

If electrolytic corrosion is a non-problem for generator installs, I wonder why Northern Lights is bringing it up as a problem that requires the compromise of safety over corrosion?

In marine environments galvanic corrosion circuits happen where ever dissimilar metals are combined because of the humidity and the salt.

Let's suppose that Northern Lights is right about the electrolytic corrosion. How would you know you have a proper boat ground on your gen neutral if the connections are all corroded?

[Frankly]

There has always been a tension/ conflict between grounding of marine electrical system and introducing unwanted corrosion. As NL indicated life safety usually wins in this argument.


The principal reason for connecting one electrical conductor (which is then defined as the neutral) to the equipment ground conductor (typically at the source) is to establish a low impedance return path. When an electrical ground fault happens the circuit protected by the breaker will immediately open. Leaves you in the dark but alive to sail another day. All this has little to nothing to do with your copper ground plate. Because ABYC requires a connection between the AC ground and the 12V negative/ engine an electrical path to the sea will usually be established. Just works out that way.


Frankly

[Locquatious]

In the end, I think I'll make the neutral to boat ground connection direct.
I'll add it to my checklist to look at and maybe sand the connections once in awhile.

[boatpoker]

Originally Posted by Locquatious

In the end, I think I'll make the neutral to boat ground connection direct.
I'll add it to my checklist to look at and maybe sand the connections once in awhile.

If I understand what you are saying (and not sure that I do because your terminology is questionable) ... that will introduce AC current into your DC negative doing that. Unwise to say the least.