Isolation Transformer - A discussion

[JohnW2]

.... I replied directly to Adrians post about a festoon on twin cable and the very common requirement to add an additional load partway along the system. Festoons are commonly used to light a pedestrian route and additional loads added at strategic points, like a floodlight or 'EXIT' sign at an access point or obstruction.

I think that you are somewhat complicating/confusing the discussion by introducing these 'moved goalpost' scenarios However ...

In this case it happens to be a classI product, Are you saying the earth connexion on a classI product should be ignored?

Well, earthing the exposed-c-ps of a Class I item would achieve absolutely nothing if the item had a floating supply.

I will not accept 'excluding a classI product from a floating supply'.

Hmm. If the item is Class I because it is not considered that the insulation ("double or reinforced") was adequate to make it safe by preventing contact with (possibly both) live conductors within, then, whether you like it or not, I don't see how it could be regarded as acceptable (safe) with a floating supply - since, as above, earthing the exposed-c-p would make it no safer.

Kind Regards, John

 

[SUNRAY]

I think that you are somewhat complicating/confusing the discussion by introducing these 'moved goalpost' scenarios However ...
Well, earthing the exposed-c-ps of a Class I item would achieve absolutely nothing if the item had a floating supply.
Hmm. If the item is Class I because it is not considered that the insulation ("double or reinforced") was adequate to make it safe by preventing contact with (possibly both) live conductors within, then, whether you like it or not, I don't see how it could be regarded as acceptable (safe) with a floating supply - since, as above, earthing the exposed-c-p would make it no safer.

Kind Regards, John

WOW!

For a start I'm not moving the goalpost, I merely replied to a question.

If I understand you correctly you are advocating totally ignoring manfacturers safety instructions.

 

[JohnW2]

WOW! ... If I understand you correctly you are advocating totally ignoring manfacturers safety instructions.

Not really - I'm saying that if a manufacturer's safety instructions (which are obviously thinking about an earth-referenced supply) indicate that the item is only safe if its exposed-c-ps are earthed, then (despite your saying that you 'do not accept' this) the product is presumably not safe/acceptable to use when the supply is floating - since, as I said, earthing the exposed-c-ps will do nothing to improve safety if the supply is floating.

Floating supplies re so relatively uncommon that they probably haven't even thought of including mention of this in their MIs.

Kind Regards, John

 

[SUNRAY]

Not really - I'm saying that if a manufacturer's safety instructions (which are obviously thinking about an earth-referenced supply) indicate that the item is only safe if its exposed-c-ps are earthed, then (despite your saying that you 'do not accept' this) the product is presumably not safe/acceptable to use when the supply is floating - since, as I said, earthing the exposed-c-ps will do nothing to improve safety if the supply is floating.

Floating supplies re so relatively uncommon that they probably haven't even thought of including mention of this in their MIs.

Kind Regards, John

I still don't see the issue here. If a fault occurs within a classI product which effectly shorts one leg of the supply to earth, what hazard does it introduce?

 

[Harry Bloomfield]

I still don't see the issue here. If a fault occurs within a classI product which effectly shorts one leg of the supply to earth, what hazard does it introduce?

I can think of one instance - If someone made contact with any earth, anywhere and addition made contact with the other pole.

 

[SUNRAY]

I can think of one instance - If someone made contact with any earth, anywhere and addition made contact with the other pole.

So talking about the 2 fault senario then?

Does this mean we're back to running a piece of faulty kit for a long time before the problem is discovered?

 

[aptsys]

Following on from a recent discussion that moved slightly off topic and into the pros & cons of isolation transformers, I thought I would start a separate learned discussion

So, the scenario :

A temporary outside decoration is to consist of a 20m length of festoon cabling with 40 coloured LED BC lamps. This a 230V powered festoon intended to be powered from the mains. This festoon string has only two cores in its mains lead - it does not have an earth core & has no provision for any kind of earthing.
!

At what point do we consider a string of 40 lamps to be more than one point? The use of an isolation transformer is generally only beneficial where just one single item is powered from it such that the risk of anything other than a single point failure meets your AFAP criteria.

 

[Harry Bloomfield]

So talking about the 2 fault senario then?

Does this mean we're back to running a piece of faulty kit for a long time before the problem is discovered?

Two sequential faults can and do happen, unless the first issue is discovered in time to prevent it. Often major disasters are the result of a whole series of issues simply not being discovered to break the chain, before they all get together to bite you on the bum.

Just saying the odds are against it are tremendous, does not really help much.

 

[JohnW2]

I still don't see the issue here. If a fault occurs within a classI product which effectly shorts one leg of the supply to earth, what hazard does it introduce?

I didn't say that it did.

It would certainly not produce an immediate hazard. However it would (probably unknowen to everyone) convert the 'floating' supply to an earth-referenced one, so that any subsequent fault would carry all the the additional hazards of having a non-floating supply.

However, you seem to have missed my main point ... IF it is deemed that an item has to be Class I (with earthed exposed-c-ps to be 'safe' (with an earth-referenced supply}) then it is NOT 'safe' to use in when the supply is floating, even if the exposed-c-ps are earthed.

Kind Regards, John

 

[bernardgreen]

There is a lot to think about when choosing / evaluating an isolation transformer. Be it high power or an audio signal isolator.

Apologies for a thrown together image.

Safety Isolation galvanic

Dual bobbins on different legs of the magnetic core provide 100% separation of the primary from the secondary, even if one winding self-destructs due to overloading the isolation is maintained long enough for safety devices or procedures to cut the supply.

Split Bobbins may fail to isolate when one winding self-destructs and melts / burn the isolating material that separates the primary and secondary winding. Isolation may fail befores afety devices or procedures can react and cut the supply

Layer wound may or may not have the Screen which should be Earthed ( supply Earth )

Without the screen safe isolation may depend entirely on the insulation on the wire in the windings and this can be abraded by vibration if the coils are loosely wound and not vacuum impregnated

Safety Isolation capacitive

Dual bobbins have the least capacitive coupling between secondary and primary
Spilt bobbins have some capacitive coupling between secondary and primary

Layer wound without the screen being Earthed can have significant capacitive coupling between secondary and primary. In the worse case this can lift the floating secondary to a potential above Earth potential and close to the potential of the Live of the supply.

When P1 ( outer layer of primary winding ) is connected to the Live of the supply then S2 ( inner layer of the secondary winding ) will be pulled by capacitive coupling towards the potential of the supply Live

This lifts the S1 potential up to supply voltage plus the secondary voltage which means 480 Volts on a 1:1 transformer or 345 Volts on a 2:1 transformer.

The amount of capacitive coupling may not ( should not ) be enough to allow a lethal amount of current to flow through a person standing on the ground or at height on scaffolding and in contact with S1 due to faulty equipment. But this current could easily be enough to create a mild shock and the person's reaction to this mild shock could result in a fall or other accident that results in injury.

 

[SUNRAY]

I still don't see the issue here. If a fault occurs within a classI product which effectly shorts one leg of the supply to earth, what hazard does it introduce?

I didn't say that it did.

It would certainly not produce an immediate hazard. However it would (probably unknowen to everyone) convert the 'floating' supply to an earth-referenced one, so that any subsequent fault would carry all the the additional hazards of having a non-floating supply.

However, you seem to have missed my main point ... IF it is deemed that an item has to be Class I (with earthed exposed-c-ps to be 'safe' (with an earth-referenced supply}) then it is NOT 'safe' to use in when the supply is floating, even if the exposed-c-ps are earthed.

Kind Regards, John

This seems to be contradiction, first you say using a cpc on the classI appliance does not introduce a hazard then you say it is not safe.

In fact you have stated it is not safe several times unless I'm totally missing your points.

 

[Harry Bloomfield]

This seems to be contradiction, first you say using a cpc on the classI appliance does not introduce a hazard then you say it is not safe

It is not installed to classI spec. if left without an earth. ClassI spec. was not intended for installation where system is not referenced to earth.

 

[SUNRAY]

It is not installed to classI spec. if left without an earth. ClassI spec. was not intended for installation where system is not referenced to earth.

Does it actually state that?
I admit I haven't delved too deeply into definitions/regs past the ... 2 barriers between live parts and the outside world...

 

[Harry Bloomfield]

However, you seem to have missed my main point ... IF it is deemed that an item has to be Class I (with earthed exposed-c-ps to be 'safe' (with an earth-referenced supply}) then it is NOT 'safe' to use in when the supply is floating, even if the exposed-c-ps are earthed.

Does it actually state that?

Near enough, yes!

 

[SUNRAY]

Near enough, yes!

Near enough, or actually stated?