Grounding generators

[Graham_Wright]

Son is part of a crew bringing back a dredger from Spain.

He is concerned to note the absence of a general earthing system.

If the generators (single and 3 phase) are not earthed, would not the volts relative to the hull float?

Perhaps there is an hidden or accidental leak between the outputs and hull.

I have asked him to make some measurements but he is now incommunicado.

 

[sailaboutvic]

Son is part of a crew bringing back a dredger from Spain.

He is concerned to note the absence of a general earthing system.

If the generators (single and 3 phase) are not earthed, would not the volts relative to the hull float?

Perhaps there is an hidden or accidental leak between the outputs and hull.

I have asked him to make some measurements but he is now incommunicado.

I was advise by FP not to Earth the generator to the boat

 

[Graham_Wright]

I can see there is no danger at 12v but at 440?

If the whole installation floats, the pd to hull could be much higher.

 

[status]

If the genny is not connected to the hull there will be no pd as there is no circuit, there can only be a pd across the conductors of the genny?

 

[Graham_Wright]

If the genny is not connected to the hull there will be no pd as there is no circuit, there can only be a pd across the conductors of the genny?

Not guaranteed. Think of the static when you leave the car. This is why grid systems have the neutral and earth connected. My feeling is that the volts will "float".

 

[mjcoon]

If the generators (single and 3 phase) are not earthed, would not the volts relative to the hull float?

My hazy recollection of my electrical machines lectures is that three-phase can be either star (which has an obvious earth point) or delta (which doesn't).

In any case there is another potential earthing point at the consuming devices. Note that in UK domestic installations the earth is part of the safety system and consumers do not earth the neutral line...

Mike.

 

[Graham_Wright]

3 phase generators produce 230 v between star and any phase and 420 volts between phases.

Consumer protection nowadays relies on the current out (L) equalling current in (N). Any difference will cause the breaker to trip. If there is a difference, it is due to current out finding a different route back. If the generator output is not earthed it could well balance out and not detect the difference.

 

[sailaboutvic]

Maybe if your or your son that worried he should just leave the boat .

 

[penberth3]

Son is part of a crew bringing back a dredger from Spain.

He is concerned to note the absence of a general earthing system...

Why doesn't he ask the ship's engineer?

 

[lw395]

Some electrical systems float completely.
It means a fault to earth at any one point is not a problem.
E.g. if you accidentally earth the live, no current flows, you just force the Neutral 230V away from earth. 2 simultaneous faults would be messy though.

 

[mjcoon]

If there is a difference, it is due to current out finding a different route back. If the generator output is not earthed it could well balance out and not detect the difference.

What other "route back" could there be? Balance is guaranteed...

Mike.

 

[Graham_Wright]

Why doesn't he ask the ship's engineer?

In electrical terms, he is the ship's engineer!

 

[Graham_Wright]

………and lose his job.

 

[Graham_Wright]

Some electrical systems float completely.
It means a fault to earth at any one point is not a problem.
E.g. if you accidentally earth the live, no current flows, you just force the Neutral 230V away from earth. 2 simultaneous faults would be messy though.

………if you accidentally earth the live - perhaps through yourself?!

 

[rogerthebodger]

IMHO when you are looking at "grounding " generators there are 2 things you need to consider

1) is the grounding of the frame of the generator. That is to ensure that the frame of the generator does not get to a voltage above the grounding point so the frame is safe to touch.

2) and more important is to detect any current leakage between the line voltage and a potential point that a person can come into contact with.

This diagram gives one method of doing that.

This also gives some more infomation

http://www.ecmweb.com/content/monitor-ground-fault-leakage-currents

 

[Graham_Wright]

IMHO when you are looking at "grounding " generators there are 2 things you need to consider

1) is the grounding of the frame of the generator. That is to ensure that the frame of the generator does not get to a voltage above the grounding point so the frame is safe to touch.

2) and more important is to detect any current leakage between the line voltage and a potential point that a person can come into contact with.

This diagram gives one method of doing that.

This also gives some more infomation

http://www.ecmweb.com/content/monitor-ground-fault-leakage-currents

That was what I thought and recommended. Unfortunately, he is out of contact now, en route for the UK (with a 3 foot freeboard!).

I trust he took my advice.

 

[mjcoon]

………if you accidentally earth the live - perhaps through yourself?!

Quite safe, until you accidentally also earth the neutral.

Mike.

 

[glynd]

How the 'pro's do it

 

[lw395]

View attachment 67056

How the 'pro's do it

Love it!

 

[lw395]

………if you accidentally earth the live - perhaps through yourself?!

Indeed.
I worked for a while in a high voltage lab where very little was earthed.
You could touch live circuits with one hand and all you would feel was a tiny current due to capacitance completing the circuit.
If you touched two different parts of the circuit you were either fried or fired.

The point is, there are several ways to not kill people with electricity.
The one we are used to is having earth bonded and neutral similar to earth.
Other systems can be better, but you have to understand the system and apply it properly.
You cannot randomly mix and match based on a limited understanding.

I'm sure there are lots of regulations that apply to ships, demanding what systems are used, but that is not my field.