Isolation transformer wiring clarification

[ChrisL]

I purchased an jemelec isolation transformer, I just want to check my understanding of how to wire.

Shoreline comes into transformer and L, N & E are wired as per input terminals in photo

The boat side coil has a live and neutral you can see in the image the neutral is linked to the earth stud, the other earth wire is for the lid of enclosure.

I am sure I’m right but just want to check I now need to take an earth wire from the stud to a stud on the shell of the boat?

final question is should the earth cable size be sufficient to carry 16A ie shoreline input?

 

[Tracy D'arth]

Then there would be no isolation!

[blackrose]

1 hour ago, Tracy D'arth said:

Then there would be no isolation!

 

Why not?

 

Surely if the boat side coil is connected to the hull earth stud then there's no direct connection to shore ?

 

I've never wired up an isolation transformer, only a galvanic isolator, but I assume the principal of making sure the mains earth on the boat side of the unit is connected to the hull is the same?

 

Hull-earth bonding is actually a separate issue to galvanically isolating a boat so the OP may find there's  already an earth connection to the hull emanating from somewhere in the consumer unit.

 

Anyway, as I said, I've never fitted an IT so wait for someone more knowledgeable to reply.

[GUMPY]

As it's wired is just about OK, the wire from the secondary neutral to the case looks like 1.5mm not 2.5mm. It would be better to take the boat earth direct to the secondary neutral and not via the stud.

One important thing to check, with the power off, is there is any resistance between secondary earth and primary earth and neutral, both readings should be open circuit. A low resistance indicates that the transformer is bypassed and will be ineffective.

[nicknorman]

10 hours ago, ChrisL said:

I purchased an jemelec isolation transformer, I just want to check my understanding of how to wire.

Shoreline comes into transformer and L, N & E are wired as per input terminals in photo

The boat side coil has a live and neutral you can see in the image the neutral is linked to the earth stud, the other earth wire is for the lid of enclosure.

I am sure I’m right but just want to check I now need to take an earth wire from the stud to a stud on the shell of the boat?

final question is should the earth cable size be sufficient to carry 16A ie shoreline input?

 

 

I think that is correct. It’s not clear what the shore earth is connected to - hopefully just a screen inside the transformer, not the transformer body or case. As mentioned, just check there is no electrical continuity between that earth wire and the case stud.

 

The earth wire should be at least as thick as the rest of the wiring . Normally it will have zero current flowing but in the event of a short circuit, hundreds or thousands of amps can momentarily flow before the breaker trips and you wouldn’t want the earth wire to vaporise before that happens.

[jonathanA]

a good source of information on these matters is the smart  gauge website here: //smartgauge.co.uk/galv_tran.html

 

Note that there is simply no connection between the shorepower earth and the earth on the boat. This is what protects the vessel from galvanic corrosion.

Note also that the Neutral and Earth are bonded on the output side of the transformer. This recreates exactly the same sitution as we normally receive from the national grid and is what allows fuses and RCDs to operate in the correct manner.

Finally note that there is a "safety screen" between the primary winding and the secondary winding which is connected to shorepower earth. This is so that any fault current in the primary (for instance from insulation breakdown) is returned to shorepower earth to trip the circuit breaker or RCD rather than electrifying the boat hull.

In effect, the transformer recreates a totally isolated AC supply just like having your own mini AC power station. Totally isolated from anythng else.

So that explains how each device protects against galvanic currents which cause galvanic corrosion. Now we need to move on to explain what has changed that now makes isolation transformers a better choice.

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[ChrisL]

5 hours ago, GUMPY said:

As it's wired is just about OK, the wire from the secondary neutral to the case looks like 1.5mm not 2.5mm. It would be better to take the boat earth direct to the secondary neutral and not via the stud.

One important thing to check, with the power off, is there is any resistance between secondary earth and primary earth and neutral, both readings should be open circuit. A low resistance indicates that the transformer is bypassed and will be ineffective.

Thanks for commenting it is actually 2.5mm to case stud from secondary connection👍

I take note of your recommendations ref testing and will make these checks 👍

Edited November 2, 2023 by ChrisL

[Onewheeler]

Is there any sort of soft start circuitry in that transformer? Without it you might well find yourself tripping the shore MCB when the power is connected.

[Tracy D'arth]

If you are connecting the case to the boat earth stud, you must ensure that the transformer case does not find a virtual fortuitous earth by way of the ground outside, otherwise you have no isolation. The boat hull will still have a virtual earth via the water, no way of avoiding this.

[nicknorman]

44 minutes ago, Onewheeler said:

Is there any sort of soft start circuitry in that transformer? Without it you might well find yourself tripping the shore MCB when the power is connected.

 

This may be true (depending on rating of transformer vs breaker and its design) but it is perhaps worth pointing out that it is random depending on the magnetised state of the core when power was removed, and the exact point in the 50 Hz waveform that the connection is made. So if it does trip, reset the breaker and try again. Hopefully after a try or two the stars will align and the thing will power up without tripping the breaker.

[Onewheeler]

3 minutes ago, nicknorman said:

 

This may be true (depending on rating of transformer vs breaker and its design) but it is perhaps worth pointing out that it is random depending on the magnetised state of the core when power was removed, and the exact point in the 50 Hz waveform that the connection is made. So if it does trip, reset the breaker and try again. Hopefully after a try or two the stars will align and the thing will power up without tripping the breaker.

Exactly! I've recently replaced my 1 kW IT (the soft-start circuitry had been on the to-do pile for several years!) with a GI so that interruptions to the shore power don't risk the MCB tripping, the charger ceasing to charge and the fridge flattening the batteries. (Also to take advantage of the new, 16A power supplies that the marina has installed). The OP's IT looks like it is just a transformer.

[GUMPY]

A suitable soft start module easy to wire in

 https://www.ebay.co.uk/itm/233814687104?

 

[ChrisL]

40 minutes ago, GUMPY said:

A suitable soft start module easy to wire in

 https://www.ebay.co.uk/itm/233814687104?

 

Thanks for link, I’ve had the transformer in my boats electrical cupboard on test in my house for a few weeks, without issues, but if I notice anything on boat I’ll look to incorporate one of these👍

[nicknorman]

1 minute ago, ChrisL said:

Thanks for link, I’ve had the transformer in my boats electrical cupboard on test in my house for a few weeks, without issues, but if I notice anything on boat I’ll look to incorporate one of these👍

When you plug the transformer into a house socket, that is probably protected by a 32A breaker. However your bollard probably has a 16A breaker.  So the fact that it works in the house doesn't necessarily mean it will work at the boat. And just for clarity, this issue can only occur when you first connect the power. (or when the power is restored after an outage).

[ChrisL]

1 hour ago, Tracy D'arth said:

If you are connecting the case to the boat earth stud, you must ensure that the transformer case does not find a virtual fortuitous earth by way of the ground outside, otherwise you have no isolation. The boat hull will still have a virtual earth via the water, no way of avoiding this.

Not sure how this could occur with my setup if I’m understanding the statement. My transformer is inside a wooden electrical cabinet and is mounted on a shelf inside cabinet so well shielded 

Perhaps the earth could move off the shoreline cable but highly unlikely 

[IanD]

12 minutes ago, nicknorman said:

When you plug the transformer into a house socket, that is probably protected by a 32A breaker. However your bollard probably has a 16A breaker.  So the fact that it works in the house doesn't necessarily mean it will work at the boat. And just for clarity, this issue can only occur when you first connect the power. (or when the power is restored after an outage).

And if there is a problem (breaker tripping) it will only happen some of the time, depending on when in the AC waveform the connection is made...

[jonathanA]

I think Tracy is referring to the situation where the IT is situated on shore (which is where it should be technically) and then the case being connected to the boat earth/hull means you risk bypassing the IT if you sat the transformer on say a muddy piece of ground or fixed it to the jetty metal piles or something like that, which connected the case to 'real earth' which ultimately is the same as the supply Earth (and neutral) via the mains supply wiring/substation.  (which is precisely the situation you are trying to avoid !)

 

in my view the case should be connected to mains earth and the transformer on shore.  The easy way to avoid all this is to put it in a plastic box rather than metal... 

Edited November 2, 2023 by jonathanA

[Tracy D'arth]

We had problems with welders tripping breakers on start up. Unfortunately on the same distribution  board were a couple of fridge freezers in the workshop that the lads used for drinks and snacks. Some of them also got into the habit of putting their frozen food shopping in there too.

If a welder was left on overnight, should not have been but we all know what happens, and the was a brief power cut, the freezers were found to be defrosting in the morning as the power could not restore itself.

[nicknorman]

9 minutes ago, IanD said:

And if there is a problem (breaker tripping) it will only happen some of the time, depending on when in the AC waveform the connection is made...

Yes, as I said earlier!

[Tracy D'arth]

1 minute ago, jonathanA said:

I think Tracy is referring to the situation where the IT is situated on shore (which is where it should be technically) and then the case being connected to the boat earth/hull means you risk bypassing the IT if you sat the transformer on say a muddy piece of ground or fixed it to the jetty metal piles or something like that, which connected the case to 'real earth' which ultimately is the same as the supply Earth (and neutral) via the mains supply wiring/substation.  (which is precisely the situation you are trying to avoid !)

 

in my view the case should be connected to mains earth and the transformer on shore.  The easy way to avoid all this is to put it in a plastic box rather than metal... 

That was my point exactly. I run isolation transformers on the work bench and they are earthed by the case to the mains input. The output is floating, and the whole transformer is in a non metallic box under the bench.

[nicknorman]

3 minutes ago, jonathanA said:

I think Tracy is referring to the situation where the IT is situated on shore (which is where it should be technically) and then the case being connected to the boat earth/hull means you risk bypassing the IT if you sat the transformer on say a muddy piece of ground or fixed it to the jetty metal piles or something like that, which connected the case to 'real earth' which ultimately is the same as the supply Earth (and neutral) via the mains supply wiring/substation.  (which is precisely the situation you are trying to avoid !)

 

in my view the case should be connected to mains earth and the transformer on shore.  The easy way to avoid all this is to put it in a plastic box rather than metal... 

Yes ideally on shore if you are an electrical purist. However having the It on the boat has many practical advantages, whereas the negative consequences are extremely improbable.

2 minutes ago, Tracy D'arth said:

That was my point exactly. I run isolation transformers on the work bench and they are earthed by the case to the mains input. The output is floating, and the whole transformer is in a non metallic box under the bench.

However your workshop is not a boat, and the IT is serving a different purpose.

[blackrose]

2 hours ago, Tracy D'arth said:

If you are connecting the case to the boat earth stud, you must ensure that the transformer case does not find a virtual fortuitous earth by way of the ground outside, otherwise you have no isolation. The boat hull will still have a virtual earth via the water, no way of avoiding this.

 

I don't really understand this. If the (metal) case of the IT isn't already connected to the terminal that the OP is connecting to the hull by the manufacturer then it should be. 

 

The case along with the boat side coil of the IT will be isolated from the shore side coil. If it isn't then it's not an isolation transformer.

[jonathanA]

49 minutes ago, nicknorman said:

Yes ideally on shore if you are an electrical purist. However having the It on the boat has many practical advantages, whereas the negative consequences are extremely improbable.

However your workshop is not a boat, and the IT is serving a different purpose.

I think the point here is that if the case it connected to the 'boat Earth' as the op has done then the transformer must be either on the boat or fully insulated (from extraneous earthing)  if on the shore.  The risk is someone doesn't understand and reads the 'purist' view and places the transformer in its nice metal box on shore.... 

 

22 minutes ago, blackrose said:

 

I don't really understand this. If the (metal) case of the IT isn't already connected to the terminal that the OP is connecting to the hull by the manufacturer then it should be. 

 

The case along with the boat side coil of the IT will be isolated from the shore side coil. If it isn't then it's not an isolation transformer.

Well I think as me and Nicknorman and tracy have all alluded to one way or another  that all depends on where the IT is placed in this case (no pun intended) and the continuity tests suggested by Gumpy would make sure it is going to function as an IT.  

[nicknorman]

13 minutes ago, jonathanA said:

I think the point here is that if the case it connected to the 'boat Earth' as the op has done then the transformer must be either on the boat or fully insulated (from extraneous earthing)  if on the shore.  The risk is someone doesn't understand and reads the 'purist' view and places the transformer in its nice metal box on shore.... 

 

Well I think as me and Nicknorman and tracy have all alluded to one way or another  that all depends on where the IT is placed in this case (no pun intended) and the continuity tests suggested by Gumpy would make sure it is going to function as an IT.  

Pretty sure in the OP’s case the transformer is being permanently installed on the boat.

[ChrisL]

2 minutes ago, nicknorman said:

Pretty sure in the OP’s case the transformer is being permanently installed on the boat.

It is inside electrical cabinet you can just see it on bottom shelf