LED Lights in bathroom over shower

[JohnW2]

Maybe I misunderstood you but I thought you were saying that, in the context of machinery, the definition of ELV stopped, and the definition LV therefore started, at 50V DC. Did I misunderstand?

Sorry, I should have said 60V DC. We don't actually define the voltages, but there are requirements that apply when using PELV to achieve protection against electric shock. Those requirements include 25 V ac and 60 V dc.

Fair enough. However, I'm still not totally clear - do you work with the concepts of "Low Voltage" and "Extra Low Voltage" and, if so, how are they defined?

Kind Regards, John

 

[stillp]

do you work with the concepts of "Low Voltage" and "Extra Low Voltage" and, if so, how are they defined?

Yes. Depends on the application, i.e. the particular standard.

 

[JohnW2]

do you work with the concepts of "Low Voltage" and "Extra Low Voltage" and, if so, how are they defined?

Yes. Depends on the application, i.e. the particular standard.

So these 'particular standards' aren't constrained to use the IEC definitions but, on the contrary, are free to use the same terms (ELV, LV and HV) as the IEC, but defined differently from the IEC ones with the same names?

If so, I think I may be temporarily speechless

Kind Regards, John

 

[stillp]

No, there are definitions for LV and ELV, one aspect of which are voltage limits. Standards writers are free to choose what they consider to be appropriate voltages within those limits.

 

[JohnW2]

No, there are definitions for LV and ELV, one aspect of which are voltage limits. Standards writers are free to choose what they consider to be appropriate voltages within those limits.

I'm very confused. I can fully understand that Standards writers are free to define their own ranges of ('safe' or whatever) voltages according to needs of the particular application, but I am amazed if they use (and are 'allowed' to use) the same names for those ranges as the IEC use, but with different voltage definitions from the IEC ones.

From what you've said, it sounds as if you would regard 55V AC as ELV, whilst IEC regards it as LV, and that you would regard 65V DC as LV, whilst the IEC regards it as ELV. Is that correct?

Kind Regards, John

 

[stillp]

What are you drinking John? It seems to have affected your comprehension.

 

[JohnW2]

What are you drinking John? It seems to have affected your comprehension.

What am I having difficulty in comprehending?

Kind Regards, John

 

[securespark]

The fact that you have fallen over and are trying to type with your toes?

 

[JohnW2]

The fact that you have fallen over and are trying to type with your toes?

Neither of those things appear to have happened, but I still don't understand what it is that stillp believes that I am having difficulty in comprehending. Do I take it that you do understand?!

Kind Regards, John

 

[JohnW2]

I'm getting even more confused. The Wikipedia (OK, not the best source of accurate information!) entry on 'ELV' (click here) includes a tabulation of the IEC definitions we are familiar with (ELV = <50V AC or <120V ripple-free DC), but also includes the following statement:

The International Electrotechnical Commission and its member organizations define an ELV circuit as one in which the electrical potential of any conductor against earth (ground) is not more than either 25 volts RMS (35 volts peak) for alternating current, or ripple-free 60 volts for direct current under dry conditions. Lower numbers apply in wet conditions, or when large contact areas are exposed to contact with the human body.

If any of that is correct, I'm now not even sure what the IEC definitions are!

Kind Regards, John

 

[stillp]

John,

The section of the Wikipedia entry that you have quoted is wrong. No great surprise there.

What you don't seem to understand is that the values of voltage that are considered to be sufficiently low to be considered non-hazardous will be different for different applications, yet all (provided they are below certain values) are considered to be ELV.
Here's an extract from IEC/TS 61201, which you might find helpful:
This Technical Specification replaces the first edition of IEC 61201 TR and provides voltage thresholds which are intended to give guidance to IEC technical committees on the selection and application of voltage limits with regard to protection against electric shock. Its purpose is to facilitate harmonization and consistency among different IEC publications. Technical
committees may use these voltage thresholds in conjunction with appropriate risk factors to set voltage limits in their product standards. Such risk factors will consider other aspects such as probability of faults, probability of contact with live or faulty parts, ratio between touch voltage and fault voltage, etc.
To estimate the type and severity of physiological effects that might be caused by electricity, the magnitude and pathway of current through a person’s body must be determined. However, from an equipment design point of view, it is advantageous to be able to predict whether unwanted physiological effects are possible or probable given only information about voltage levels on accessible conductive surfaces. If the maximum available voltage is sufficiently low under the expected circumstances to be unable to cause enough body current to cause unwanted physiological effects, then the safeguards normally required to avoid the occurrence of these physiological effects may be reduced or eliminated. Voltages below critical levels that
are unlikely to be hazardous in this respect have normally been called Extra-Low Voltage (ELV). Based on this information Technical Committees may wish to review their defined values of Extra-Low Voltage.
The text might have changed slightly, this is copied from a working draft.

 

[JohnW2]

John, The section of the Wikipedia entry that you have quoted is wrong. No great surprise there.

Fair enough - as you say, no great surprise!

What you don't seem to understand is that the values of voltage that are considered to be sufficiently low to be considered non-hazardous will be different for different applications ...

As I have said repeatedly, I fully understand and accept that bit, but ...

yet all (provided they are below certain values) are considered to be ELV.

This is where I'm getting both confused and surprised. I can fully understand "the voltage considered to be safe in application X" varying between applications (different Xs) but, if this is what you are saying, I can't understand a specific term like "Extra Low Voltage" being defined as having different meanings (values) according to the application.

Is that what you are saying, or are you perhaps saying that there will be different definitions for safe voltages for different applications (which is fine) and/but they will all be called ELV if they are also within the range of a common (say the IEC) definition of ELV?

Remember that this all started because you questioned my statement that 90V DC was (per IEC definition) ELV.

Here's an extract from IEC/TS 61201, which you might find helpful:

Thanks. The final bit seems to perpetuate my uncertainties:

Voltages below critical levels that are unlikely to be hazardous in this respect have normally been called Extra-Low Voltage (ELV). Based on this information Technical Committees may wish to review their defined values of Extra-Low Voltage.

This does, indeed, seem imply that different Technical Committees might produce their own, different, definitions of "ELV" (rather than just a safe range of voltages in relation to their area of interest) - which is the concept which rather concerns me. Is that your interpretation of what it means?

Kind Regards, John

 

[stillp]

are you perhaps saying that there will be different definitions for safe voltages for different applications (which is fine) and/but they will all be called ELV if they are also within the range of a common (say the IEC) definition of ELV?

Remember that this all started because you questioned my statement that 90V DC was (per IEC definition) ELV.

Change "will be called ELV" to "might be called ELV" and yes, that's what I was saying.
Yes, that is how it started, with my mistake in not checking that the BS7671 usage of the term ELC was the same as that with which I usually work.

Think of ELV as a means of protection against harm by electric shock, rather than a fixed numeric value, and you might feel more comfortable.

 

[JohnW2]

Think of ELV as a means of protection against harm by electric shock, rather than a fixed numeric value, and you might feel more comfortable.

Well, yes, I would probably feel a bit more comfortable, but:

(a)... I (and I suspect most others) have never thought of ELV, LV and HV as being anything other than IEC-dictated strictly numerical definitions of ranges of voltages, and ...

(b)... If we are to have a situation in which these terms can have different (numeric) meanings in different situations/contexts, it is surely essential that the terms be qualified so as to ensure that they are interpreted correctly??

It's perhaps a bit ironic that a lot of this discussion in occurring in the context of Standards, since I would have thought that one of the most fundamental aspects of Standards is that they should ensure consistency and non-ambiguity of definitions and terminology!

Kind Regards, John

 

[stillp]

It's perhaps a bit ironic that a lot of this discussion in occurring in the context of Standards, since I would have thought that one of the most fundamental aspects of Standards is that they should ensure consistency and non-ambiguity of definitions and terminology!

Yes, mine do, it's the other b*****s that get it wrong!

But seriously, they are unambiguous (although I still can't find the basic definitions, that's because I've other things to do) but they are not used in the sense of, for example "voltage shall not exceed ELV" but more like "when protection against electric shock is done by the use of ELV, the voltage shall not exceed X volts".

Yes, it does seem that there are people who think they can be given a numeric limit that will be adequate in all applications, but engineering is not that simple.
They are "IEC-dictated strictly numerical definitions of ranges of voltages", but within those ranges the responsible Technical Committee will select a value that is appropriate for the product/application for which they are responsible.