Isolation in Full Bridge Transformers?

[24vtingle]

We have multiple offline 360W Full Bridge (Currnt doubler output) transformers to prototype.
The circuits that these go into will at all times be handled by trained people.
So it doesn't matter that full mains isolation is not present.

(ie we don't margin wind and no Triple ins wire)

ETD44 Np/Ns = 36/16, Vin = 400V, Vout = 60V
Np = 2 strnds 36 turns of 0.315mm ECW
Ns = 4 strnds of 16 turns of 0.315mm ECW

We have so many of these and none of them have flashed over in several months.
However, we still want to do what we can to increase the mains isolation.
As such, we are thinking of spreading some silicone over the primary (round the edges) before we wind the secondary over the top of it.
We don't want varnish as that will set and we want to be able to easily unwind and rewind these formers for something else at a later date.
Do you think the silicone will get between the coils (pri and sec) and provide just that extra bit of isolation?

We have a layer of transformer tape between pri and sec layers but as you know round the edges the sec and pri coils can
be very close to each other. (even though we do try and have as much as possible tape separation)

The application just cant use TIW. We are often just not likely to be able to get hold of it, and so must just get on without it.

So what do you think about a wee bit of silicone round the edges of the pri coil?
(surely it can't make matters any worse? Do you have better ideas?)

 

[Easy peasy]

I wonder if the Dept of Labour in your country reads these posts ?

 

[24vtingle]

Thanks, i havent actually said what country here, its not necessarily my country in question.

 

[Easy peasy]

Respectfully - most countries have basic laws about selling things for profit that are less safe than is practicably realise-able - a good thing to keep in back of mind.

Public endangerment springs to mind . . .

 

[FvM]

I'm missing a description of output circuit. Transformer insulation requirement can be functional, basic or reinforced, depending on instrument isolation coordination. If the insulation is safety related, part of a safety barrier between mains or other HV circuit and exposed circuit, it must be designed as reinforced or basic + supplementary insulation.

The designation of exposed circuit doesn't depend on "trained people" operating it.

 

[24vtingle]

Its for a test and all involved are like me and trained people who regularly work with live mains.

The designation of exposed circuit doesn't depend on "trained people" operating it.

Ayk, many engineers working with offline power supplies work with exposed live mains on the bench.

 

[FvM]

O.k., what's the objective of your question at all?

--- Updated Apr 27, 2025 ---

An insulation without safety function is called functional. In turn, functional isolated secondary circuit must be treated as if it carries mains voltage.

 

[24vtingle]

Thanks, i suppose another option would be to tape wrap the stranded enamelled copper wire. The yellow transformer tape sounds best for this?
Mylar? (Its amazing how often an idea comes to you when you get it "out of your head" and ask a question about it.)

Tape.

https://multimedia.3m.com/mws/media/1949722O/3m-polyester-film-electrical-tape-56-data-sheet.pdf

 

[24vtingle]

Hi,
Presumably since margin winding is legal and satisfies the regs for isolation, then presumably winding tape round one of the coils at the end bits is also satisfactory?.....since its the same principle of isolation by tape. The tape between the layers would take care of the insulation between the coils in the middle bit of the coil bobbin.