Symbols

[sparkticus]

Many thanks. It seems to require application of a rather strange type of logic to get to the meaning of by having two instances of the symbol which denotes [a]!

I agree and I am glad they don't write the clues for crossword puzzles (or do they for the Times? ) It makes me wonder about "any" logical thought process of the people who came up with such a system. To be fair it is possibly the result of "harmonizing" a range of different standards and symbols (though that may be being too fair)

As an aside, here is a list of xformer symbols.

http://www.google.co.uk/#hl=en&suge...gc.r_pw.&fp=9bc2c12fe7b1c385&biw=1012&bih=730

 

[sparkticus]

As an aside, here is a list of xformer symbols.

https://docs.google.com/viewer?a=v&...&sig=AHIEtbRYIz-niQKH34bpzA6F0FCTYaRYKQ&pli=1

 

[JohnW2]

As an aside, here is a list of xformer symbols.....

Many thanks. The symbols for "short-cirecuit proof" and "non short-circuit proof" transformers ('separating' or 'isolating') seem frighteningly similar, particularly given how small these symbols may be when applied to components!

... and, as a matter of interest, what is the difference between a 'separating' and 'isolating' transformer? I presume from the words that both are proper (not auto) transformers, so it's quite hard to imagine what the distinction might be!

Kind Regards, John.

 

[sparkticus]

... and, as a matter of interest, what is the difference between a 'separating' and 'isolating' transformer? I presume from the words that both are proper (not auto) transformers, so it's quite hard to imagine what the distinction might be!

As I understand it,

separating transformer: a transformer in which the primary and secondary windings are electrically separated with basic insulation.

Isolating transformer: a separating transformer (as above) with a protective type of partition between the primary and secondaries.
(that is an approved partition with characteristics specs listed under EN61558-2-6 and one or two other EN/UL conformities - too many documents!!!!)

safety isolating transformer: a separating transformer designed for SELV (seperated ELV) or PELV (protected ELV) applications.


EDIT
safety isolating transformer: an isolating transformer designed for SELV (seperated ELV) or PELV (protected ELV) applications.


EDITsafety isolating transformer: an isolating transformer which includes those designed for SELV (seperated ELV) or PELV (protected ELV) applications.

 

[sparkticus]

what on earth is meant by "short-circuit proof (both inherently and non-inherently"?!

non-inherently would have some sort of thermal or other cutout to prevent significant heat rise under SC conditions (would open circuit the secondary and sometimes, double pole, the primary)

Inherently would allow for temperature rise within the transformer design for smaller transformers and for larger ones might include a "magnetic shunt" They are usually tested under SC conditions and maximum measured winding temperature recorded and marked as operating temp. The SC temperature is obviously higher than normal load temperature but not high enough to present risk of significant internal/external thermal effects.

 

[JohnW2]

separating transformer: a transformer in which the primary and secondary windings are electrically separated with basic insulation.
Isolating transformer: a separating transformer (as above) with a protective type of partition between the primary and secondaries.
(that is an approved partition with characteristics specs listed under EN61558-2-6 and one or two other EN/UL conformities - too many documents!!!!)

Thanks. That makes sense - essentially just a question of the degree of 'assured insulation' between primary and secondary.

Some of these things seem to have 'SELV' on them, but without any of the transformer/isolation symbols. Does 'SELV' alone guarantee anything in terms of the degree of isolation provided?

Kind Regards,
John

 

[JohnW2]

non-inherently would have some sort of thermal or other cutout to prevent significant heat rise under SC conditions (would open circuit the secondary and sometimes, double pole, the primary)
Inherently would allow for temperature rise within the transformer design for smaller transformers and for larger ones might include a "magnetic shunt" They are usually tested under SC conditions and maximum measured winding temperature recorded and marked as operating temp. The SC temperature is obviously higher than normal load temperature but not high enough to present risk of significant internal/external thermal effects.

Thanks. I suppose that makes sense in terms of the word 'inherently'. Do I take it that you are saying that 'inherently short-circuit proof' means that it can cope indefinitely with a short circuit across the secondary without heating to a dangerous extent?

Kind Regards, John.

Kind Regards, John.

 

[sparkticus]

Thanks. That makes sense - essentially just a question of the degree of 'assured insulation' between primary and secondary.

Exactly.

Some of these things seem to have 'SELV' on them, but without any of the transformer/isolation symbols. Does 'SELV' alone guarantee anything in terms of the degree of isolation provided?

I believe (will confirm tomorrow) SELV does (by design criteria) limit the output voltage to 50VAC and provide safe isolation between primary and secondary. Interestingly in a PELV design the earth is connected to the secondary (more interestingly on TN-C-S supplies)

 

[sparkticus]

'inherently short-circuit proof' means that it can cope indefinitely with a short circuit across the secondary without heating to a dangerous extent?

Yes, the transformer would be continuously rated for the SC current. They are often "higher impedance" transformers since the ohmic properties of the secondary winding material is designed/selected to aid in current limiting (that is in smaller transformers) There is of course a trade-off when the load in poorly matched - though wide enough for powering a range between 2 - 6 12V/35watt halogens for example.

 

[JohnW2]

I believe (will confirm tomorrow) SELV does (by design criteria) limit the output voltage to 50VAC and provide safe isolation between primary and secondary.

That certainly seems implicit in the definition, but I was wondering whether the 'SELV symbol' constitutes a guarantee of the isolation, as would the appropriate transformer symbol.

Interestingly in a PELV design the earth is connected to the secondary (more interestingly on TN-C-S supplies)

Indeed. I suppose your 'more interestingly' comment would really only become of any great importance if, say, PELV were taken outdoors, in which case the infamous possibility of significant potentials between the TN-C-S 'earth' and true earth would become an issue. In any other context I can think of, I'm not sure that the SELV/PELV distinction is of any great importance, at least in domestical installations - after all, in is an inherent characteristic of such installations is that there are plenty of bits of touchable metal which are connected to the installation's 'earth'.

Kind Regards, John.

 

[sparkticus]

That certainly seems implicit in the definition, but I was wondering whether the 'SELV symbol' constitutes a guarantee of the isolation, as would the appropriate transformer symbol.

Yes, just looked that up. the acronym "SELV" is defined by the IEC as having (at least) the following characteristics:

-maximum output 50VAC/(120VDC ripple free)
-protective separation between primary/secondary winding/circuits.
-no return path via ground/earth
-at least a specified minimum distance between conductors and insulation barriers

I suspect that manufacturers place additional symbols in an effort not to be excluded by volume buyers/procurement people or perhaps for reasons of defining where the unit can be placed/secured (the double M-inverted triangles for example)

 

[sparkticus]

Indeed. I suppose your 'more interestingly' comment would really only become of any great importance if, say, PELV were taken outdoors, in which case the infamous possibility of significant potentials between the TN-C-S 'earth' and true earth would become an issue. In any other context I can think of, I'm not sure that the SELV/PELV distinction is of any great importance, at least in domestical installations - after all, in is an inherent characteristic of such installations is that there are plenty of bits of touchable metal which are connected to the installation's 'earth'.

Yes an outdoor experience is what I was thinking though I have to say that other than PC PSUs some AV/TV equipment I have not noticed PELV very often if at all.

 

[JohnW2]

Yes, just looked that up. the acronym "SELV" is defined by the IEC as having (at least) the following characteristics: ...

Thanks. I presume therefore that writing 'SELV' on a product is meant to guarantee that it does possess those characteristics?

Kind Regards, John.

 

[JohnW2]

Yes an outdoor experience is what I was thinking though I have to say that other than PC PSUs some AV/TV equipment I have not noticed PELV very often if at all.

Indeed - no other obvious examples come immediately to my mind, either - but, as I said, unless I'm missing something, I really can't think of any problem with PELV indoors in a domestic setting.

Kind Regards, John.

 

[sparkticus]

Yes an outdoor experience is what I was thinking though I have to say that other than PC PSUs some AV/TV equipment I have not noticed PELV very often if at all.

Indeed - no other obvious examples come immediately to my mind, either - but, as I said, unless I'm missing something, I really can't think of any problem with PELV indoors in a domestic setting.

No I can't either, at least not a real world example. It is just that am "sensitised" (as you will know from previous posts) to anything that is connected to earth where there is no practical risk of it becoming live. I tend to evaluate (for better or for worse) the risk of the item becoming live VS the risk of connecting it to earth. My perceived risks of connecting it to earth:
1) If someone comes into contact with a live item while holding an "earthed" conductive item then a PD could be dropped across them.
2) An earthed conductive item may rise in potential momentarily during a fault condition. The fault condition may be remote from the earthed item. A SELV system does not present that small risk assuming the SELV PSU meets all requirements. A PELV system introduces that risk albeit that it is one end of the ELV (secondary) that is actually connected to earth (including the transformer barrier)

Having said all of that, I do not hold a (and do not want to appear) dogmatic view because ultimately I always try to take the common sense approach, it is simply that "these things" go through my head during my evaluation process. In reality I doubt if an uninstructed person can come into contact with the live conductors of a PELV system by mistake anyway. I just need to convince myself of that on a case by case basis.