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Class II with Chassis earth bond?

Former Community Member
Former Community Member
The floating output of a Class II product, if measured to Earth may present a voltage of 80-100V AC, The reason is that there is a small capacitance connected between the output side (low voltage output) and the input side (mains) of a switch mode power supply from the safety approved Y-capacitors.

The limits for this leakage current is internationally set to safe values for humans (<250uA). The Voltage is therefore regarded safe to use, and expected.


My preference in such situations therefore to isolate the electrical parts from any user accessible metal parts, to prevent  users noticing a tingle when touching them.


My questions relates to using a bond from the accessible metal parts to a safety earth instead.


1) The earth is not required for safety, only comfort but could it affect leakage current?


2) The application is a pre-existing installation, an LED strip powered from a class 2 supply with the led tape mounted to a metal extrusion which itself is mounted to a metal container. If the extrusion were to be earthed to the metal enclosure, would the assembly then be treated as class I or remain class II ? 


3) Which standard(s) should I be referring to find these answers.



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    I understand the desire to avoid the slight tingles, on back-of-finger or a slight cut, that <0.25 mA can give.  Keeping these parts away from the users and lightly insulated from exposed metal seems a good way to achieve that, if it means very little extra cost.


    I doubt that it's a good idea in a product to include a connection to the 'PE' of the plug or other connection just for this purpose. If you have a product that's supposed to be produced and sold in bulk for profit, then can you really afford to add to the cost without solid advantage?  I guess that very few users would experience the shocks, and they probably wouldn't be put off the product; and having earthed metalwork is not necessarily a safety benefit (more chance of getting a good earth connection if getting shocked from some other part; and more chance of danger if the PE itself gets a dangerous potential); and extra wires and connections cost money and might need more product-testing; and for other markets (continental) where small 2-pin plugs could be used it means further expense in plugs and possible difficulty for places where earthed sockets aren't available (light-fitting supplies in some countries).  Basically, you would seem to be increasing the cost and doing little or nothing to improve customer attraction. People often don't buy on a technical basis.


    1)  I suppose the leakage current would be higher and consistent, when going to a solidly earthed frame instead of through occasional body contact including possible shoes, wooden floor, etc.  But it would be around the 250 uA or less, anyway [Edit: that is, provided that the class-II design already limited leakage to around this much - or 500 uA as GK suggested (later post) might be permitted.]


    2) Good question!  I've no idea.  Well - having an earth wire would mean it looks like Class I, but if it doesn't need it for safety it should be Class II.  Perhaps you'd only be allowed the earth if claiming it's needed for safety. In some regulations, the green-yellow colour should only be used when for safety purposes; do you need to call it a functional earth...? Most philosophical: I'll await others' views. 


    3) Definitely not something I can answer. Nor most people in a "wiring regulations" forum rather than product standards, LV-directive etc.  But there's at least one regular here who almost certainly will know all of this!  


    Concluding:  having a bit of insulation from the supposedly safe leakage-limited parts to touchable metal seems the economic and simple method to avoid occasional tiny shocks.  Failing this, probably just let it be unless your product is analagous to Bentley/RR/etc in its field.



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    Quite a lot of modern electronics does not really fall neatly into class i or ii, in that it has a CPC, and this may connect  to exposed parts, but the construction of insulation on internal live parts,  windings on transformers and so on is such that it could be declared class II if the CPC were omitted.

    A number of laptop power supplies are like this. They are Class I, and exceed the minimum requirements. Just because having an earthed metal case permits you to have bare spade terminals inside carrying mains does not mean you have to.

    Equally touching a class 2 item with an earthed wire is very much like doing so with an earthed finger, and is perfectly acceptable.

    The main example I can think of where you are actually advised to do so is the earthing of the outer of the coax on TV and radio antennas, and then when you plug the coax cable  into the TV set you are ameliorating the 'tingle' risk from the fact the TV set has a double insulated construction.
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    Andrew James Skinner:

    The floating output of a Class II product, if measured to Earth may present a voltage of 80-100V AC, The reason is that there is a small capacitance connected between the output side (low voltage output) and the input side (mains) of a switch mode power supply from the safety approved Y-capacitors.

    The limits for this leakage current is internationally set to safe values for humans (<250uA). The Voltage is therefore regarded safe to use, and expected.


    My preference in such situations therefore to isolate the electrical parts from any user accessible metal parts, to prevent  users noticing a tingle when touching them.


    My questions relates to using a bond from the accessible metal parts to a safety earth instead.


    1) The earth is not required for safety, only comfort but could it affect leakage current?

    This depends whether the conductors of the LED strip are actually connected to the metal or not, but if it is, you might get the 250 μA as an AC. The limit for "leakage" on Class II equipment is usually about 0.5 mA in product standards.

    2) The application is a pre-existing installation, an LED strip powered from a class 2 supply with the led tape mounted to a metal extrusion which itself is mounted to a metal container. If the extrusion were to be earthed to the metal enclosure, would the assembly then be treated as class I or remain class II ? 



    Class I and Class II relate to the Low Voltage (mains) supply.


    The term SELV (Separated Extra-Low Voltage) is used to describe an extra-low voltage source for which the output is electrically separated from the mains (in BS 7671, this is less than 120 V DC or 50 V AC in dry conditions, although some standards have lower limits for SELV, at 24 V DC and 12 V AC, so that the products may be used in environments which may become wet or damp). This is typically done by supplying it from a source similar to one of the following:
    • An independent battery

    • A safety isolating transformer or a source offering equivalent isolation, such as a motor-generator, opto-isolated electronics, etc


    PELV is where an SELV source is used, and remains separated from live conductors (Line and Neutral) of the low voltage supply, but one conductor is connected to protective Earth.


    If your LED driver is equivalent to SELV, the conductors of the tape are insulated from the metal extrusion, it remains SELV.

    If one of the conductors in the tape becomes connected to Earth, then it is PELV.


    If the LED driver is not adequately separated, or has higher voltages than permitted by the product standard for SELV, it is classed as FELV, more research is required, as it may be designed to provide protection by electrical separation.
    3) Which standard(s) should I be referring to find these answers.

    The base standard that discusses protection against electric shock, and the terms we're looking at, for both products and installations, is BS EN 61140.


    The standard for LED controlgear is BS EN 61347-2-13.


    The standard for many other common electronic products is BS EN 62368-1 (supersedes BS EN 60950-1). Unlike BS EN 60950-1, BS EN 62368-1 does not use the terms SELV and PELV, instead considering something called ES levels - ES1 may well be equivalent to SELV and PELV, but not always in wet conditions.

     


  • Former Community Member
    0 Former Community Member
    Thank you for the detailed replies.

    Firstly I believe the -ve contact of the LED strip is coming into contact with the metal extrusion to which it was mounted, causing the tingling affect.

    I believe PELV (“an extra-low voltage system which is not electrically separated from Earth but which otherwise satisfies all the requirements for SELV”) if a bond to Earth is added to the extrusion.

    This is muddied however by compliance of the LED driver in the existing installation:

    The used constant Voltage LED Driver’s (MW LPV-100-24) data sheet previously specified EN60950-1, but now shows EN62368-1 approved (updated 04/03/2020), but does not explicitly state SELV.

    I would have assumed SELV by virtue of the 60950-1 in the past, but am not sure I can say the same for EN62368-1 going forward

    That said, the design is old. The declaration of conformity was signed Dec 2019, so components and assemblies that complied with EN60950 were still acceptable and could be used as part of equipment certified to EN 62368-1 without design change.  The block diagram on datasheet still shows magnetic and opto isolation between input and output. So it is likely that product is SELV

    In a new installation the simplest solution would be to insulate the LED strips or specify a known SELV supply.

  • 0
    Indeed I have had a slight tingle from a SELV LED power supply, end of a hot day, very sweaty paws. Unlikely to be an issue in actual usage. Simply insulate all the joints to prevent incidental contact and it's very very unlikely to be an issue
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    Andrew, apologies for butting in with a supplementary question. Is the so called leakage current a different concept to the protective conductor or touch current? In the COP for In-service Inspection and Testing of Electrical Equipment a maximum of 0.25mA is permitted or double that if the equipment has no control device or is fitted with a control device which provides 3mm clearance in the off position.  That is in-service, I know, but it would seem to tally with what Graham said. However, I was under the impression that the protective conductor current was measured with a simulated body resistance of 1000ohms.
  • 0
    lyledunn:

    Andrew, apologies for butting in with a supplementary question. Is the so called leakage current a different concept to the protective conductor or touch current? In the COP for In-service Inspection and Testing of Electrical Equipment a maximum of 0.25mA is permitted or double that if the equipment has no control device or is fitted with a control device which provides 3mm clearance in the off position.  That is in-service, I know, but it would seem to tally with what Graham said. However, I was under the impression that the protective conductor current was measured with a simulated body resistance of 1000ohms.


    Isn't that "touch current" Lyle?


    Protective conductor current in terms of BS 7671 is protective conductor current from whatever source (i.e. it's undefined) and therefore includes current collected from EMI absorbed by shielding, and "leakage currents" from interconnected equipment, as well as standing protective conductor currents from noise filters on the mains, and "leakage" across insulation and transformers.


    Product standards may define protective conductor current measurements and/or touch current measurements - the touch current measurements being in the way you have defined (it's a combination resistive and capacitive network to simulate a dry body impedance).


  • 0
    I have this every day. I have a class 2 TV, as most are. I have a class 1 Audio system which I sometimes use for the TV sound (much better frequency response and quality). I plug the Hi fi into the headphone jack, which is connected to the isolated voltage of the "chassis" as we used to call it. So I have a class 2 equipment with an earth connection. The only problem is during plug in I have to remember not to have the amplifier turned on as the floating AC voltage makes it output a very loud hum, probably driven by several volts AC at least (the hi fi is quite powerful at least 120 dB SPL). This is not particularly dangerous electrically but does assalt the ears and partner! A solid Earth to the TV would be a good idea. The TV would still be class 2 isolated but the PSU output would be Earthed. No problem, mains isolation is better than required and the appliance still meets class 2 requirements but has a 3 core mains cable, the mains voltage is still isolated by double or reinforced insulation.

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