Isolating Transformer earth output

[Fuss]

My 230v Isolating Transformer has 150v on the earth output. Can anyone explain why this figure come about?.

The setup is as follows...
230v In, 230v out.
Input blue,brown earth connected to Inputs in transformer
Output blue,brown earth connected to Outputs in transformer

There is the option to tie down the output earth to the output neutral which solves the problem but my question is that when the earth is floating why the figure of 150v?

[daddle]

If a terminal is truly "floating" it can be any voltage. What you see is perhaps the average of all the tiny leakages inside the transformer. It would be more interesting to know what the current is when this terminal is connected to ground.

I assume this is a "safety ground" for the transformer. As such it should be grounded.

[IceDog]

If your Grounding system (lets say a ground wire tied to the engine block) is not connected to the blue output lead of the isolating transformer, then that blue lead is not a neutral since by definition, a neutral is an intentionally grounded conductor. What you have instead is two ungrounded (hot) conductors, one of which may have a grounding connection downstream through the frame of a piece of equipment etc.

When you say the Earth output of the transformer has 150V, I assume that you mean you are measuring 150V between the transformer ground terminal which would be tied to the frame of the transformer, and one of the ungrounded conductors.

When you establish a neutral on the boat side of the transformer by grounding one of the transformer legs, you can expect to read 230V from hot to neutral, or hot to ground (because the meter completes a path from hot, through the low impedance ground and back through the neutral to the supply.

When your ground is floating, and you read from an ungrounded hot wire to ground, a circuit somehow has to be completed from the hot supply wire from the transformer through the meter to the ground system and return to the supply hot on the other side of the transformer coil. Otherwise you will read zero volts. With your floating ground reading 150V you are measuring part of a series circuit between one ungrounded hot from the supply back to the other. Current is flowing from the hot through the meter leads and meter, through the ground terminal/frame of the transformer, through the ground system trying to find a path back to the other side of the supply (transformer). It is finding a path but its not a low impedance path such as it would be if that side of the transformer was intentionally grounded. Rather, it is finding a connection between the grounding system and the wiring from the transformer through the various devices. 80V is being dropped along that path (230V-150V). That path potentially includes (get it, potentially...that was a play on words) any ac grounding conductor or path, any DC negative or ground path, any bonding conductors or path and the path from any connected device in the system back to the supply transformer.

American NEC and ABYC standards (I'm not an expert on ABYC standards) require a separately derived system such as our isolating transformer to have one of the transformer taps grounded in order to establish a new grounding system and allow sufficient fault current to flow back to the source through a low impedance path so it can trip a breaker. This creates a Neutral or more properly a grounded conductor. European and other standards may be relying on a residual current device (measuring the difference between incoming and outgoing current ) to ensure the safety of people and livestock. Perhaps that is why the standards differ?

The problem with this stuff is that sometimes the attempted answers only serve to confuse more...

[Lancerbye]

You say you have 150 volts on the floating earth ground wire if I understand you correctly. What is the reference point you are using to make this measurement? If it is floating and you are measuring with a high input impedence meter like most digital meters you need a reference or you will be measuring induced voltage which is meaningless and has no current capacity.

[IceDog]

Here is a simplified discussion of a floating ground arrangement for a power supply. It may help even though it is not boat specific. In the US shore electrical systems, the neutral is required by the NEC to be bonded to the ground at the transformer or as far downstream as the first disconnecting means of the system. Isolated grounding for equipment is treated as a separate issue when required to prevent ground loops and transient voltages.

[tuberider]

On european boats they have a 230v three wire system, that is when you ground one side,the blue wire, of the transformer.on In the states you have a four wire system, there you don't ground on side of the transformer, you put the white neutral and the green ground on the frame of the transformer. You don't need a neutral wire on the imput side when you use a transformer. Now if you have a 150v input, then you take one side of the output of the transformer and tie it to the frame of the transformer with the green wire.

[Lake-Effect]

Depending on the transformer unit... if they included some small-value bypass caps or transorbs (to absorb spikes) and depending on the measurement setup, you can get miniscule currents across those caps which are measurable.

[edit] could also be paths through connected equipment. Was the transformer still in circuit, with connected equipment? [/edit]

I always cringe at this kind of question, and some of the answers. I don't think the answerers are necessarily wrong, but unless they have the time to produce a detailed response with drawings and references, it's often hard to get the point across to an absolute beginner... which you have to assume the OP is, unless you know more about him/her.

To the OP, it would help alot if you could post the make/model of the transformer, a scan or link to the transformer's manual, and a photo of your install.

The problem is... if you ground the transformer wrong:
1) you could die
2) your boat could kill a swimmer
3) you might have some bits of your boat corrode faster than normal

#3 is your own problem, #1 & 2 are ours. And your insurance company's.

Rather than a forum, it's advisable to have someone local take a look.

(electrical/electronic is one of my fields)

[Lake-Effect]

Good article on AC neutral on boats:

Demystifying the Neutral-to-Ground Connection - Professional BoatBuilder Magazine

This article elaborates on the ABYC standard.

Key bit:

The connection between the AC safety grounding wire and the neutral wire must be made only at the power source—when and only when the unit is supplying power to the vessel.

[Fuss]

Thanks for all the replies.

So now that I'm down on the boat I have tied the output neutral to the output earth and all is as normal.. No volts between neutral and earth. There is a jumper in the victron for this purpose to make the rcd trip.
Thanks again for all the replies

[donradcliffe]

Just make sure there is no connection between the input and output grounding wires. Its also worthwhile to put a clamp-on ammeter around the shore power cable and make sure it reads zero when you are using shore power.

[Fuss]

Originally Posted by donradcliffe

Just make sure there is no connection between the input and output grounding wires. Its also worthwhile to put a clamp-on ammeter around the shore power cable and make sure it reads zero when you are using shore power.

Thanks Don.. I did those checks and all ok. Shore power cable reads 0 so no power being lost to an alternative path