Symbols

[JohnW2]

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.

Yes, as we've discussed many times here, that's a totally logical approach. Indeed, with the occasional very cranky exception (haven't seen him for a while!), I don't think anyone disagrees with that approach. If/when there are any arguments or disagreements, it's in relation to evaluation of that risk-benefit balance in a particular situation. Of course, if one has properly created an equipotential zone within a building, this discussion should be moot as far as 'indoors' is concerned (give or take discussions about damp walls!). It's when one gets outside (and can't create such a zone) that one has to start thinking and/or worrying.

It's interesting that I have only really thought in this way for the last decade or two. Prior to that, essentially through lack of thought (and less certainty about equipotential zones), I think I probably regarded it as 'intuitively obvious' that 'everything' should be earthed - and I suspect I was not the only one to think like that at the time.

Kind Regards, John.

 

[JohnW2]

...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:

I've just noticed what might be a little 'wriggling'. Some of the ones I have bear the label "SELV EQUIV.". I wonder waht that means in comparison with 'proper SELV'?

Kind Regards, John.

 

[ban-all-sheds]

When this is all done and dusted, could someone please write a wiki article with symbols & explanations?

 

[JohnW2]

When this is all done and dusted, could someone please write a wiki article with symbols & explanations?

Good idea. With a bit of luck we may find one or more websites that cover it sufficiently comprehensively between them that most of it could be done by links.

Kind Regards, John.

 

[matt1e]

It's interesting that I have only really thought in this way for the last decade or two. Prior to that, essentially through lack of thought (and less certainty about equipotential zones), I think I probably regarded it as 'intuitively obvious' that 'everything' should be earthed - and I suspect I was not the only one to think like that at the time.

Kind Regards, John.

good point John
I remmember when the 15th came out the advice of my mentors was at the time"if its metal, earth it"

Matt

 

[JohnW2]

good point John. I remmember when the 15th came out the advice of my mentors was at the time"if its metal, earth it"

Yep, that's how I was brought up, too. I suppose it was, in some senses, a precursor of more formal consideration of equipotential zones - although we were talking and thinking about 'earthing' everything, it was probably actually most valuable as an exercise in bonding - the earthing itself was probably of secondary importance, the more valuable thing being that by 'earthing everything' we were actually bonding everything together.

Kind Regards, John.

 

[sparkticus]

I've just noticed what might be a little 'wriggling'. Some of the ones I have bear the label "SELV EQUIV.". I wonder waht that means in comparison with 'proper SELV'?

I suspect (but do not know for sure) that it is an electronic SELV PSU. Most do have some electronics in them anyway (and not to say that a xfrmer is not electronic) but all have to assure isolation between primary and secondary potentials so even an "all electronic" SELV PSU needs a method of isolation since semiconductors (no matter how many used) are specifically excluded as a way of providing isolation in a SELV PSU.

I seem to have a lot of documentation on SELV and switching PSU topologies and will try to find a way of posting it.

 

[sparkticus]

It's interesting that I have only really thought in this way for the last decade or two. Prior to that, essentially through lack of thought (and less certainty about equipotential zones), I think I probably regarded it as 'intuitively obvious' that 'everything' should be earthed - and I suspect I was not the only one to think like that at the time.

Yes, there was a time when I would have "blindly" bonded everything in sight though I was not an "electrician" as such at that time. Up until around 20 years ago I worked in the semiconductor industry where great attention is paid to "earthing" and equipotential zones because semiconductor materials (especially gate oxide layers) can be "punched through" by electro-static discharge (ESD) So we had to ensure that everything was tied back to a main earth via a high resistance (typically a 1 Mohm resister) in an effort to provide a "soft discharge" path. In some cases we simply made sure that some surfaces where floating. We obviously tried to prevent the devices accumulating a charge in the first place but some processes such as ion-beam doping, SEM and FIB systems tend to do just that by nature. So when I re-trained as an electrician I sort of "went in" with that way of thinking.

 

[JohnW2]

I've just noticed what might be a little 'wriggling'. Some of the ones I have bear the label "SELV EQUIV.". I wonder waht that means in comparison with 'proper SELV'?

I suspect (but do not know for sure) that it is an electronic SELV PSU. Most do have some electronics in them anyway (and not to say that a xfrmer is not electronic) but all have to assure isolation between primary and secondary potentials so even an "all electronic" SELV PSU needs a method of isolation since semiconductors (no matter how many used) are specifically excluded as a way of providing isolation in a SELV PSU.

The one I have in front of me (with 'SELV EQUIV.') certainly is an 'electronic' unit (a 12V constant voltage LED driver) - but, as one might expect, it does appear to contain wound transformers (two in fact, unless one is a choke) - which I imagine are working at a relatively high frequency (relatve to 50Hz!) and provide the required input/output isolation.

Kind Regards, John.

 

[sparkticus]

The one I have in front of me (with 'SELV EQUIV.') certainly is an 'electronic' unit (a 12V constant voltage LED driver) - but, as one might expect, it does appear to contain wound transformers (two in fact, unless one is a choke) - which I imagine are working at a relatively high frequency (relatve to 50Hz!) and provide the required input/output isolation.

Yes and of course an LED driver needs to output DC at (and as you say) constant voltage for LEDs in parallel or constant current for LEDs in series. I think you are right on there being a transformer inside as the only practical way of providing true isolation from primary to secondary circuits though that tends also to be a feature of "switch mode psu" I suspect the frequency is quite a few KHz. There are a range of standard design topology's like that.

 

[sparkticus]

When this is all done and dusted, could someone please write a wiki article with symbols & explanations?

Apologies, I missed this. I'll be happy to post something if no one else is working on it? It will be a few days before I can get to it though.

Is there any secret to making things stick in the wiki? Any authorisation required?

 

[JohnW2]

Apologies, I missed this. I'll be happy to post something if no one else is working on it? It will be a few days before I can get to it though.

That would be useful.

Is there any secret to making things stick in the wiki? Any authorisation required?

If you wanted to create a'sticky' posting, I think you'd have to get the mods to achieve that for you. However, as far as the wiki is concerned (which is a different matter), they are not 'sticky' (except that old versions are saved) - anyone can modify (or potentially delete) a wiki entry. I've only ever edited existing wiki entries - never created one from scratch - but I suspect if you go to the wiki, it is probably fairly obvious how you do it. I'm sure that others here will be able to hope.

Kind Regards, John.

 

[sparkticus]

Thanks, I had not taken on board that the the "sticky" and "wiki" were two different things. In that case I'll attempt to create a wiki.

 

[JohnW2]

Thanks, I had not taken on board that the the "sticky" and "wiki" were two different things. In that case I'll attempt to create a wiki.

It's probably easy. What I do remember is that, when I edited a wiki entry, modifying a table was a bit of a mission, but I managed eventually - ISTR that it used a somewhat unfamiliar (to me) mark-up language. However, I think you can probably create wiki pages with just plain text and images, provided that you don't need any clever formating. I'm sure that people here will be able to help if you get stuck - let us know how you get on.

Kind Regards, John.

 

[ban-all-sheds]

Object reuse.

Find a Wiki article with a table in it, and use it as a template.