Isolating Transformers, The Earth wire connection conundrum

[goboatingnow]

Originally Posted by Oceanride007

To go boating now.
Don't know anything about your reverse polarity lights issue, other that on a ship you would check shore connection EVERYWHERE if you had motors on board or single pole switching.
Earth lamps are what makes Neutral earth viable on ships. otherwise more moon rocks would be more common.
I think we are getting concensus on the other matters, The situation called a conundrum by the OP is spot on, this situation is unique in that the issues haven't been sorted already, and some of ask What to do.
I see your point on ground loops but I wonder if such a neutral point could be isolated.

I'm the OP actually.

My comment re reverse polarity , ie live an neutral reversed was that any installation on a boat should be safe irrespective of incoming polarity. An IT solves this problem beautifully , but systems can be rendered safe even in non IT situations .

Dave

[Oceanride007]

Personnally I would go the Isolated Transformer way, but there are some of us who insist on RCD devices and lots of work place safety requirements mandate RCD devices to even work on a yacht. I think I've identified a niche in how this can be done, I put it to the test here.
To make my suggestion about isolated neutral point, I can see its a bit oxymoron. What I meant was that your plug in AC tools and appliances are connected thru its earth wire to a neutral point created between two matched resistors connected to the outputs of the IT. No connection to Engine, hull, earth its floating. Your RCD Can be connected in the normal way.

[chala]

Originally Posted by pbiJim

Dave,
Are you saying that an appliance can not leak current to ground simply because it's power source is the floating secondary winding of an isolation transformer?

In the case of a refrigerator that has a steel shell that happens to touch some metal part of the boat, if the compressor motor leaks some current to the metal shell, would that not be an example of current leaking to ground?

If a hot plate were also in use & the other power leg leaked current in that appliance, would that not create a hazard for a person holding a pan to fry some eggs while touching a metal surface in the boat, or reaching to grab another egg out of the refrigerator?

My concern would be that just because a secondary winding was not grounded at the transformer, does not mean that an appliance down stream can not create a ground loop there.

Pb you are right, you understand what Jedi and GoBo and other are unable to understand. In clear a refrigerator may be live with part of hot2 and the pan may be live with part of hot1. Any one getting in contact with both will be shocked. The protective device will see no inbalance, just an increase in load current. It is why earth wire and RCD’s where created.

Originally Posted by goboatingnow

In a true floating system, ie dual hots and an IT , RCDS on the secondary side provide very very little if any protection , in a floating system , since all currents including fault currents must return to the IT , the rcd will do little.

If a full wiring situation where the secondary side earth wire is connected to appliances cases and return to the one of the IT secondaries , in essence establishing a local " pseudo " hot and neutral system , RCDS can now detect imbalances and trip. However such wiring systems are in themselves trade offs.

In a full floating system RCDS do virtually nothing

Dave

Gobo make up your mind.

[sailorchic34]

Originally Posted by chala

Pb you are right, you understand what Jedi and GoBo and other are unable to understand. In clear a refrigerator may be live with part of hot2 and the pan may be live with part of hot1. Any one getting in contact with both will be shocked. The protective device will see no inbalance, just an increase in load current. It is why earth wire and RCD’s where created.
.

You are correct that if someone comes in contact with both sides of the IT circuit they will get shocked. However a ground will not prevent that on a IT secondary as both legs are isolated from the primary side and isolated from ground. That is one of the main reasons to have a isolation transformer after all. You can hold on to one leg and jump in the water and you would not get shocked.

Extending the ground from the land side to the boat will not effect safety as there is no voltage between either leg and ground. Potential exists only when both sides of the secondary coil are in circuit. Its not two wires, its two ends of the same wire.

[Oceanride007]

To Chala
I can see you have a legit concern against IT. Would my floating earth suggestion (above) resolve that. And I think you can still hold on to one leg and jump in the water, but I wouldn't.

[sailorchic34]

Originally Posted by Oceanride007

To Chala
I can see you have a legit concern against IT. Would my floating earth suggestion (above) resolve that. And I think you can still hold on to one leg and jump in the water, but I wouldn't.

With an single phase isolated transformer installed on a steel ship for example, how do you think the floating ground would provide protection. The two legs of the IT secondary coil are isolated from the primary side and from ground. Just wondering....

The only circuit in a IT secondary is from leg to leg and nothing leg to ground.

Going back to the OP's original diagram, the ground shown tied to one leg of the secondary is not a ground. Its just another hot leg and will not provide any protection.

There is no current flow in the secondary, unless both legs of the secondary are in circuit. One leg only has no voltage potential at all. So you can hold on to it with no danger what so ever.

Sorry I'm only a professional engineer and being a woman too, I hardly know anything at all. Better use small words too.

[sailorchic34]

Looking at some of the older posts I can see there is some confusion due to geographic locations. Chala being down under has a 220 volt system with two hot wires. Very much like the find folks in the EU.

We in the US, have to be different and have a 120v system.

The difference is Chala's and EU's have two hot legs with a voltage potential to ground from either leg.

in the US, 120V single phase has a hot wire, a neutral wire (a center tap on the secondary leg) and a ground wire. For all intents, both neutral and ground are at 0V. You can grab the neutral or ground bus in a US 120V panel and your completely safe. Do that down under and they will place you down under.

For residential service in the US, we have two legs of 120V 180 degrees out of phase giving 240 leg to leg and 0v neutral leg/center tap and ground. For marine 30 amp service its 120V, single phase. This for recreational boats.

So Chala is correct as far as he goes. So is everyone else.

Remember we talking world wide and what is standard and code in one spot can be completely different elsewhere.

[pbiJim]

Originally Posted by goboatingnow
Chala, I'm afraid you haven't the faintest idea of what you are talking about , an isolating transformer with fully floating outputs has no earth connection, ie earth is not in the circuit. Hence there can be NO earth leakage from an appliance.


Originally Posted by pbiJim

Dave,
Are you saying that an appliance can not leak current to ground simply because it's power source is the floating secondary winding of an isolation transformer?

In the case of a refrigerator that has a steel shell that happens to touch some metal part of the boat, if the compressor motor leaks some current to the metal shell, would that not be an example of current leaking to ground?

If a hot plate were also in use & the other power leg leaked current in that appliance, would that not create a hazard for a person holding a pan to fry some eggs while touching a metal surface in the boat, or reaching to grab another egg out of the refrigerator?

My concern would be that just because a secondary winding was not grounded at the transformer, does not mean that an appliance down stream can not create a ground loop there.

Originally Posted by goboatingnow

An appliance fed by a fully floating supply from an IT cannot leak current to earth, earth is not in the circuit , what you are relating are faults in an earth referenced or earth return orientated mains supply system , like the one into your house. In that case the earth is a valid return circuit , which is why you can get a shock.

Hence your last paragraph cannot and does not apply.

Dave

Perhaps I did not communicate effectively.

In my example, earth becomes part of the circuit via a faulty appliance, not because it was put there intentionally. Hence, my last paragraph does apply.

[sailorchic34]

Originally Posted by pbiJim

Originally Posted by goboatingnow
Chala, I'm afraid you haven't the faintest idea of what you are talking about , an isolating transformer with fully floating outputs has no earth connection, ie earth is not in the circuit. Hence there can be NO earth leakage from an appliance.


Originally Posted by pbiJim

Dave,
Are you saying that an appliance can not leak current to ground simply because it's power source is the floating secondary winding of an isolation transformer?

In the case of a refrigerator that has a steel shell that happens to touch some metal part of the boat, if the compressor motor leaks some current to the metal shell, would that not be an example of current leaking to ground?

If a hot plate were also in use & the other power leg leaked current in that appliance, would that not create a hazard for a person holding a pan to fry some eggs while touching a metal surface in the boat, or reaching to grab another egg out of the refrigerator?

My concern would be that just because a secondary winding was not grounded at the transformer, does not mean that an appliance down stream can not create a ground loop there.



Perhaps I did not communicate effectively.

In my example, earth becomes part of the circuit via a faulty appliance, not because it was put there intentionally. Hence, my last paragraph does apply.

On a boat with a 120V (or 220V) single phase isolated transformer there is no ground or ground loop. Even if that appliance is connected to the shore ground or a ground plate on the hull, there can be no current flow from either leg of the IT to shore ground. Current flow ONLY happens leg to leg. Even with multiple outlets / loads there is no current flow from leg to ground as there is no circuit path leg to ground with an isolated transformer.

[sailorchic34]

Here's a primer for ya'll

Electrical Safety: Systems and Devices

[pbiJim]

Originally Posted by sailorchic34

On a boat with a 120V (or 220V) single phase isolated transformer there is no ground or ground loop. Even if that appliance is connected to the shore ground or a ground plate on the hull, there can be no current flow from either leg of the IT to shore ground. Current flow ONLY happens leg to leg. Even with multiple outlets / loads there is no current flow from leg to ground as there is no circuit path leg to ground with an isolated transformer.

Apparently, I am still not making myself clear.

In my example, Appliance #1 has a fault from L1 of the ac system to it's case, which happens to be touching a metal part of the boat. This causes an unintentional grounding of L1 via a fault.

Appliance #2 then has a fault from L2 to an exposed surface.

Together, these two faulty appliances cause a dangerous condition, despite the added protection of the isolation transformer.

I was not talking about shore ground. I was only talking about local ground on the boat itself, such as an engine block, keel, or other part that happens to be metal.

[Tantalus]

Originally Posted by pbiJim

Apparently, I am still not making myself clear.

In my example, Appliance #1 has a fault from L1 of the ac system to it's case, which happens to be touching a metal part of the boat. This causes an unintentional grounding of L1 via a fault.

Appliance #2 then has a fault from L2 to an exposed surface.

Together, these two faulty appliances cause a dangerous condition, despite the added protection of the isolation transformer.

I was not talking about shore ground. I was only talking about local ground on the boat itself, such as an engine block, keel, or other part that happens to be metal.

Ya know....you guys are actually agreeing...just using different vocabulary.

[FlyingCloud1937]

I think everyone is missing the the key point (well not everyone)

"A boat floating in water is always at some potential to earth ground".

We are not taking about some science experiment mounted on rubber blocks isolated to 1000 volt potential, if it's a boat floating in water.

Almost every boat that I know of, has an on-board battery charger operating when plugged into shore, either with or with out an ISO-TRANS....That one devise can and almost always is the fault path between sea and shore. Now factor in what is bonded or potentially bonded on the DC side.

Now factor in that many also have inverter/chargers. An inverter by fault bonds DC/AC

Now factor in various of bonding systems that are either faulted, or designed improper.

And now we have a potential ...safe electrics are by design to protect against an UN-known fault.

If a fault potential was always known, we wouldn't have to protect against it.

Lloyd

An ISO-Trans is only legit to the point it saves metal, it fails in life saving.

[goboatingnow]

Several posters simply don't understand IT basics. There are several ways to configure IT. The one were are talking about is a fully floating system. This is similar to units used in hospitals.

In a full IT setup. It would be advisable to still fit an RCD and the usual breakers on the secondaries . No earth wire is installed

Lets examine the potential fault scenarios

(1) Single live fault to appliance case , ( ie battery charger etc ) this does not generate any shock hazard , however no fault is triggered either

(2) both live wires shorted to an appliance , depending on the potential ground or other paths. A short may generate and the breaker trips

(3) one live faulted to appliance A and the other live faulted to appliance B. a person touching both appliances will be inserted into the circuit and receive a shock. No protection will be extended. However this is a very unlikely fault situation.

The solutions are

( a) re establish ground referenced output and earth wire and rely on breakers and RCDs. Hence the IT provides no security as most shock hazards are fault through human to ground

(B) establish a fault protective wire , connecting one live of the IT to appliance case with an RCD then ahead of the common protective wire and IT secondary.

This wire must not lead to inadvertent ground connection , but it does protect a live fault to appliance as in sceanario (1)

I called it a conundrum for a reason.

It depends on what you regard as the safety issues on board a boat. I personally agree with Nick , in that most shock faults are exposed live or appliance faults to ground via the human. A floating IT solution completely eradicates that issue, even though the existence of such a fault is not easily detectable.

Every system has fault scenarios that cause problems , its a balance of options

dave

[goboatingnow]

Originally Posted by FlyingCloud1937

I think everyone is missing the the key point (well not everyone)

"A boat floating in water is always at some potential to earth ground".

We are not taking about some science experiment mounted on rubber blocks isolated to 1000 volt potential, if it's a boat floating in water.

Almost every boat that I know of, has an on-board battery charger operating when plugged into shore, either with or with out an ISO-TRANS....That one devise can and almost always is the fault path between sea and shore. Now factor in what is bonded or potentially bonded on the DC side.

Now factor in that many also have inverter/chargers. An inverter by fault bonds DC/AC

Now factor in various of bonding systems that are either faulted, or designed improper.

And now we have a potential ...safe electrics are by design to protect against an UN-known fault.

If a fault potential was always known, we wouldn't have to protect against it.

Lloyd

An ISO-Trans is only legit to the point it saves metal, it fails in life saving.

With an floating IT solution a faulty battery charger cannot generate a shock hazard by generating a fault path to ground. Simply even if such a path was created ( which is extreme unlikely, ) fault current cannot flow throug a human in that case because earth is not on the circuit ( an issues many people here seem to find difficult to understand ) the existence of a path to earth does not mean current can flow in it.

Dave