SELV PELV & FELV

[pompeygit]

I understand SELV & PELV but can somebody give me an example of FELV that does not include the attributes of the former?

 

[Grizzly]

Any use: http://en.wikipedia.org/wiki/Extra_low_voltage#Types ?

 

[plugwash]

One example might be an electronic device that used a transformerless powersupply. The voltages should be low in normal operation but a lost neutral or reversed polarity could leave the whole circuit live.

 

[NotHimAgain]

You have asked for an example that does not include the attributes of SELV or PELV but FELV does include many of these attributes.

i.e: All have extra low voltage sources.

The difference between the systems lies in the specification for these sources. For illustration let's restrict our consideration to transformers sources.

For each system the transformer secondary voltage must not exceed extra-low-voltage. Now for SELV and PELV the transformer has to meet a specified standard of construction - in particular the likelihood of the primary voltage appearing on the secondary circuit due to, say, an inter-winding fault, must be minimised.

This requirement is often met by winding primary and secondary on separate bobbins or by including a copper inter screen between windings that are wound over each other.

FELV does not require this and 'any old transformer' will do. A typical example of a FELV circuit is a transformer supplied door bell system.

 

[ban-all-sheds]

Or even no transformer at all.

Give me a 52.32 ohm resistor rated at 1kW and I'll build you a FELV supply capable of powering a 12V 50W lamp.

 

[NotHimAgain]

Or even no transformer at all.

Give me a 52.32 ohm resistor rated at 1kW and I'll build you a FELV supply capable of powering a 12V 50W lamp.

Oh dear - still you know what they say about a little knowledge .

Its like the former science teacher who made his own electric fence using a 'dropper resistor' - it would be funny but a child was killed.

Read 411.7.4

 

[ban-all-sheds]

NHA - I never claimed that it would comply with the regulations, or that it would be safe.

Just that it would be a functional ELV supply.

 

[NotHimAgain]

NHA - I never claimed that it would comply with the regulations, or that it would be safe.

Just that it would be a functional ELV supply.

I see - so following your advice could be dangerous then - I must remember that

 

[ban-all-sheds]

Can you show me where I advised anybody to do that?

 

[NotHimAgain]

Ah so you were not giving advice - my apologies.

However you did offer to build it for the OP. So maybe it's your offers of help I should be wary of .

 

[ban-all-sheds]

You can stop worrying - that will be of no practical concern to you.

 

[JohnD]

old Radar and TVs used to have Mains Droppers like that. You can tap off various low voltages. Dangerous if you poke around inside though.

 

[ban-all-sheds]

Uninformed poking around inside equipment like that can bring you surprises which are far more exciting than low-voltage dropper resistors...