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PELV isolation requirements for 3 Phase device
frank 11001215401
Joined 22/05/2020 - 6 Posts
Question
hello,

   we are designing a device that is connected to 3 phase 230Vac rms. the low voltage part needs to be connected to protective earth (since outgoing signals need to be referenced to PE) - i think this means the low voltage part is PELV

   this device also measures the voltages of each phase (L1, 2 and 3) with respected to Neutral. our measurements are based on a voltage divider reducing the 230Vac rms to about 0.25Vac peak over a sense resistor. we have 4 meg resistance to the L1,2 and 3 (from the sense resistor) but would prefer a low resistance to Neutral (from the sense resistor)

   my question is: what resistance is required from Neutral to the PELV circuit? (i read 0.5 meg minimum on "live conductors" but no where can i find anything particular for neutral isolation)

thanks
frank
the netherlands
 
12 Replies
davezawadi (David Stone) 28267643
Joined 26/06/2002 - 423 Posts
Frank
I would like to discuss this a bit further but the forum is not really suitable. Could you send me an email to davezawadi@yahoo.co.uk and I will start a conversation. For complex reasons I need a degree of secrecy to this discussion of a fairly sophisticated electronic problem.


 
frank 11001215401
Joined 22/05/2020 - 6 Posts
we are using the allegro micro acs71020, https://www.allegromicro.com/en/products/sense/current-sensor-ics/zero-to-fifty-amp-integrated-conductor-sensor-ics/acs71020

reason:
- compact (hall, no ct's)
- digital interface
- accurate (i & v sampling)

complex part is using 3 of those in a 3 phase system...
mapj1 80733779
Joined 22/07/2004 - 1901 Posts
edit
IF all you want to do is to measure phase to earth voltages or at least a divided version of them with a high impedance op amp or ADC or similar, use a 2 capacitor string as a voltage divider, and use caps rated for that use small value Y2 devices are a good start.
If you are still stuck tomorrow, I can try sketching something, but bed is calling.

The  more sophisticated ones look much the same, bigger relays. Personally I'd like to see force guided relay and the second contact used to make sure the armature has actually moved - welded contact detection, but I've yet to see it done well.  Value engineering pushes the other way.
video of one being opened
frank 11001215401
Joined 22/05/2020 - 6 Posts
well that is a very primitive evse...

most of those are not even obeying the spec, not proper earth detection, no "diode" check on the cp, no relay checks

we do have an proper isolation on the low voltage part. just figuring out how to do this on the power metering

thanks
frank
mapj1 80733779
Joined 22/07/2004 - 1901 Posts
It may help to look for inspiration at some pics from inside a typical EVSE this is a 'granny lead' but you will see not an extra insulating  barrier or earthed screens to achieve SELV or PELV separation from the mains - all pure Functional ELV
Instead see how the ELV relay wiring and mains are all mixed in the same enclosure, with no barriers or anything - the enclosure and cable insulation ratings providing the protection rated to the highest voltage present covering the lot. Yes pilot currents are referred to the mains CPC, but that does not make it the rest of it PELV. (it does mean the supply generating the pilot currents should be transformer derived though - either at 50Hz or more likely a switcher at something between 25KHz and 100KHz..)
If I did want to get a data connection out of that to user equipment that was not so protected (and USB seems to be the modern classic, not so long ago it was RS232 folk got shocks off...) I'd have to split the PCB and have a creepage gap between the isolated user stuff and the mains side ELV  stuff.
If it really is only a few LEDs and a button, it is much easier to put the LEDs behind a window, so if there was a fault, they could become live and no-one would be hurt, and use a mains rated push button even if for extra low voltage for the same reason.


Take care with that phrase  'low voltage'  - to many of us, that includes  230V phase to neutral and 400v phase to earth. 'High' voltage starts somewhere
above a 1kV. If you really mean less than 50V, it is called Extra Low, in the UK at least.
 
frank 11001215401
Joined 22/05/2020 - 6 Posts
hi mike

   i thought PELV comes with the way it is all connected? i no not need it to be PELV, just want it to work and be safe... (and pass certification tests)

   the device will have terminals on the low voltage side. these connect to the ev (charging protocol) and possibly an external button or led light

   the sensing resistors are connected to power metering chips (they also measure current through hall effect) - the circuitry was just taken from the datasheet. the load to measure is the ev charging

thanks
frank

 
mapj1 80733779
Joined 22/07/2004 - 1901 Posts
OK so far. So, why do you need PELV ?
- are there exposed conductive parts connected to the Vin monitor that will be handled by a human operator ( screw terminals, metal bodied connectors, headphone jack USB etc ?) If the answer is no, or these can be isolated from the mains connected stuff, you do not have a problem.
I hope you are omitting a shed load of protection components from your circuit that really are there, or it will not survive first use in the thunderstorm or beside an electric railway (the two have common aspects.)
Also be aware that any real device will have an input capacitance that will conspire with the megohms (and stray capacitance from wiring around the megohms) so that you will not get a sensibly flat frequency response - are you just interested in the main 50Hz component, or are you wanting to see distortion of the cycle, due to perhaps thyristor controlled loads or similar ? If the latter you may want to think of it more like a 'scope probe.
frank 11001215401
Joined 22/05/2020 - 6 Posts
a little more on the actual sensing part:

in the schematic below: Vin is the 230V ac from L1, 2 or 3. VinP and VinN are inputs to the voltage measurement chip. VinN is internally biased to half the chip voltage

since we have a 3 phase system we have this 3x. due to the internal biassing this can only work if all 3 are separately powered with a isolated power supply (too expensive) or if all VinN are connected together. in that case the large resistance to Neutral will distort the measurements (only giving the "correct answer" = the desired ac voltage over the sense resistors when the amplitudes of L1,2 and 3 are equal)

so the plan is/was to have 2x 2 meg resistors to L1,2 and 3 and no resistance to Neutral - this works just fine. but may not be possible due to safety regulations as i now understand...

could add the minimum resistance to neutral (2x 1 meg?) and add capacitance to the connected VinNs with respect to something stable (PE?)

it is an interesting puzzle and i appreciate your input!


i
 
mapj1 80733779
Joined 22/07/2004 - 1901 Posts
Is this PELF, or just FELV - is the insulation on the exposed ELV parts not up to mains voltages? (bell wire or similar ?)

Normally things that are referenced to Live or neutral in this way are considered FELV, and then boxed up as if containing live parts (an earthed metal box, or double insulation /re-inforced insulation)
For instrumentation monitoring mains (data loggers etc) we would normally AC couple (series capacitance X or Y rated) and some resistance as well.
It is also possible as another approach if you need the DC terms, to have an ADC and micro-contoller that is in effect insulated as if it is all flying at mains potential and then opto couple serial data out of it to another one that is truly PELV or even SELV.
If the flying deck voltage is many kV Ac off earth we may even use an optical fibre link to keep the ELV we touch with USB connectors and so on distinct from the ELV we try not to - in my world that may be up a transmitter mast or inside a box with  an EHT unit.

Are you able to explain your purpose a little more?
frank 11001215401
Joined 22/05/2020 - 6 Posts
hi andy
   
   thanks for the info. parts are getting more clear to me now

   the product is an evse (electric vehicle supply equipment). the relevant spec for us is IEC 61851-1_2017 i think. in its most basic form an evse switches on L1,2,3 and N only after a number of safety checks are done (and an ev is detected...)

   from what you are saying i guess we need the same isolation resistance (in the mega ohm range) from N as from L1,2 and 3 -- which is too bad as we need a reference to Neutral. i guess we can can some filtering after the resistance

thanks
frank
 
gkenyon 82239607
Joined 07/05/2002 - 925 Posts
frank:
hello,

   we are designing a device that is connected to 3 phase 230Vac rms. the low voltage part needs to be connected to protective earth (since outgoing signals need to be referenced to PE) - i think this means the low voltage part is PELV

   this device also measures the voltages of each phase (L1, 2 and 3) with respected to Neutral. our measurements are based on a voltage divider reducing the 230Vac rms to about 0.25Vac peak over a sense resistor. we have 4 meg resistance to the L1,2 and 3 (from the sense resistor) but would prefer a low resistance to Neutral (from the sense resistor)

   my question is: what resistance is required from Neutral to the PELV circuit? (i read 0.5 meg minimum on "live conductors" but no where can i find anything particular for neutral isolation)

thanks
frank
the netherlands
 

Neutral is a "live conductor", and therefore the separation (electrical separation) is the same as a Line conductor (which are also "live conductors"). This is because in certain fault conditions, the Neutral can rise in potential to any of the Line conductors.

A single resistance to a Line (or Neutral) conductor in an LV circuit may not meet the requirements of SELV or PELV - protective separation to live conductors of LV circuits is required.  I would suggest you look at the relevant product standard for the requirements for the minimum requirements for your particular product, as they do vary (and some standards are moving away from "SELV" and "PELV" to Energy Levels, e.g. ES1, ES2 etc. in EN 62368-1). If it's measurement equipment, this might be EN 61010-series, but if it's for a protective or electricity metering function, other standards may apply instead. If it's for something that's classed as Machinery in line with the Machinery Directive, EN 60204-1 is likely to apply, and the requirements are similar to BS 7671 / HD 60364-series (and hence other EU national wiring codes).

Unfortunately, without knowing the classification of the product, and therefore the relevant standard(s), it's not possible to advise further.

AJJewsbury 77361768
Joined 13/08/2003 - 1433 Posts
what resistance is required from Neutral to the PELV circuit? (i read 0.5 meg minimum on "live conductors" but no where can i find anything particular for neutral isolation)
The term "live conductors" in BS 7671 includes neutrals as well as line conductors.

Generally PELV would need a safety source (such as an isolating transformer) - I doubt simple resistors, even high value ones, would be sufficient on that count, as there's the matter of reliability as well as design values. BS 7671 has particular requirements for SELV and PELV sources - but your situation might come under some equipment standard rather than wiring regs.

I know some voltage sensing systems use an isolating transformer (e.g. 230V to 9V) and then read the voltage on the 9V side and scale up to deduce the original voltage.  Otherwise it might be better to consider the system FELV rather than PELV.

   - Andy.

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