Understanding the Grounding of Transformers in Marine Electrical Systems

[Junior01]

Hi,
can somebody help me please.
I have situation like on picture in link. So, marine electrical system 440V, transformed to 230V for heater. But, my question is, why transformer is grounded on such way. I don't understand this grounding. thank you.

 

[FactChecker]

One side of the output should be grounded. That seems like a natural way to do it. Is there anything specific that you disagree with or want to ask about?

 

[Junior01]

Why it is necessary to ground one side of secondary? What is the line between primary and secondary which is also grounded and why it is grounded?

 

[Baarken]

Why it is necessary to ground one side of secondary?

I think this video would be of interest for you, @Junior01, it explains different ways of grounding the secondary side of a distribution transformer.

 

[FactChecker]

Things that are well grounded are safe to touch, so it is good to have as much grounded as possible. It is also necessary to control what is grounded and make it easy to keep track of the ground.
Suppose two devices are made with no ground. Each one could have a fault where the chassis connects to the opposite wire. If you touch them both, you will be shocked. Providing a tracked ground connected to each chassis means that you can always touch them both safely. It also makes sure that if the hot wire touches the chassis, then a breaker can detect it and trip.

So to specifically answer your questions: One side is grounded so that you know there is a gound provided and can keep track of ground. The "wire" between the primary and secondary is probably a metal form that both primary and secondary wires are wrapped around and is mounted in the transformer chassis. So it should be grounded.

 

[Junior01]

So to specifically answer your questions: One side is grounded so that you know there is a gound provided and can keep track of ground. The "wire" between the primary and secondary is probably a metal form that both primary and secondary wires are wrapped around and is mounted in the transformer chassis. So it should be grounded.

Thank you, but if the "Wire" is wrapped around metal form, and this metal form is grounded it would means that we have a "ground fault", because phase and Earth are directly connected via this metal form?

 

[FactChecker]

Thank you, but if the "Wire" is wrapped around metal form, and this metal form is grounded it would means that we have a "ground fault", because phase and Earth are directly connected via this metal form?

Oh, the wires are insulated (or at least one of them). There is no electrical connection in the winding.

 

[sophiecentaur]

Why it is necessary to ground one side of secondary? What is the line between primary and secondary which is also grounded and why it is grounded?

It is quite possible to have the secondary of a transformer 'floating' The problem is that a 'negligible' high resistance path from the primary side can leave you with the mean voltage of the secondary connections floating at a high voltage. This is a shock risk and can be avoided by a connection of one of the secondary terminals to earth. It defines the range of voltages that can exist in the secondary circuit (never greater than the nominal secondary volts)..
That line between primary and secondary windings probably refers to a screen between the two windings. The screen is wrapped round the inside winding and stops any damage to the transformer creating a Primary-secondary connection because the fault will be connected to earth. Of course, the screen must not be a closed loop or you would have a shorted turn.
An isolating transformer with floating secondary has its uses, though, because it protects the user from a reversed L and N in the supply.

 

[wirenut]

Are you referring to the line between the two windings that has the ground on the end? If so that indicates the core of the transformer is grounded, In the USA at least the chassis (and thus the core through a strap) must be grounded to building steel. Also if there is a neutral formed on the secondary, it (x0) is also connected to the transformer chassis (unless you have a corner grounded delta, a whole other animal).

 

[Asymptotic]

Although not relevant to your grounding question, very often both transformer primary leads are fused in the event the primary winding shorts either turn-to-turn, or to ground.

 

[sophiecentaur]

Imagine wearing a foetal heart monitor and do your usual day to day work?

Really? I thought that double pole fusing was not approved of these days. A few decades ago, we went through all our imported equipment, eradicating all neutral fuses.
I can understand some of your reasoning but doesn't it increase the risk of a fault being undetected?

 

[jim hardy]

There is no electrical connection in the winding.

Not sure quite what that was meant to convey.

@Junior01

One point on the transformer winding SHOULD be 'grounded' ( i prefer term "earthed" ) for two reasons:
1. So static charge cannot build up on the winding (plus whatever it powers) and pierce the insulation.
2. So if insulation fails in another part of the circuit, short circuit current will blow the fuse and apprise you something needs to get fixed.

That "PE" wire is probably "Protective Earth" and 'earthed' back at whatever transformer supplies the 3 phase power.
Its job is two-fold
1. To conduct fault current from failed insulation back to its source so a fuse will blow or a breaker will trip.
2. Accomplish (1) with little enough voltage drop that someone touching the short circuited equipment won't get electrocuted.

You would benefit greatly from reading IEEE's "Green Book" , standard 142 . Unlike so many standards this one is written in easy to understand language and its first few chapters explain the basic concepts of grounding very clearly.
After you've read "The Green Book" , the "National Electric Code" starts to make sense. Surely somebody in your engineering department has a copy you could borrow - it's a small book only about 1/4 inch thick. You can get the basics in an evening.

Good luck and keep on asking that's how we learn.

old jim

 

[sophiecentaur]

After you've read "The Green Book" , the "National Electric Code" starts to make sense. Surely somebody in your engineering department has a copy you could borrow - it's a small book only about 1/4 inch thick. You can get the basics in an evening.

After you have read those two you may need a bit of a sit down and a jar of the beverage of your choice. (But some people do it for pleasure! Cheers @jim hardy .)

 

[FactChecker]

Not sure quite what that was meant to convey.

One point on the transformer winding SHOULD be 'grounded' ( i prefer term "earthed" ) for two reasons:

That was in response to the suggestion in post #6 wires winding around the core would short out. At least that is how I interpreted it.

 

[jim hardy]

That was in response to the suggestion in post #6 wires winding around the core would short out. At least that is how I interpreted it.

i didnt understand #6 myself... my post was more toward Junior01 and I've clarified that with a late edit. No offense meant..

Thanks !

old jim

 

[dlgoff]

1. So static charge cannot build up on the winding (plus whatever it powers) and pierce the insulation.

I recently ran into electrostatic shields in an isolation transformer (single phase) project. Here's a link for downloading a pdf file about them and here's a screen shot from it: