Creepage and clearance of through hole resistor

[eepty]

Recently I found a resistor broken on a PCB. It is a MOF type 1/2W through hole resistor. I found that the resistor is directly placing (touching) on a PCB Live trace and I know that the potential difference between the resistor and the trace is about 220Vac. There is a small hole on the resistor and a burning mark on the PCB trace. Therefore I presume an electric arc was formed between the resistor and the PCB.

Then I read the safety standard EN60335-1. For the part describing creepage and clearance, it has a remark: "Lacquered conductors of windings are considered to be bare conductors" As I know common MOF resistor body is just a lacquered coating,

I would like to ask is this resistor considered as "Lacquered conductors of windings"? If so this is violate the safety standard.

If this is not, is a MOF through-hole resistor body touching a Live PCB trace violate the standard?

Appreciate if you can provide strong supporting document or information.

Thank you very much

 

[mjuneja]

Recently I found a resistor broken on a PCB. It is a MOF type 1/2W through hole resistor. I found that the resistor is directly placing (touching) on a PCB Live trace and I know that the potential difference between the resistor and the trace is about 220Vac. There is a small hole on the resistor and a burning mark on the PCB trace. Therefore I presume an electric arc was formed between the resistor and the PCB.

Then I read the safety standard EN60335-1. For the part describing creepage and clearance, it has a remark: "Lacquered conductors of windings are considered to be bare conductors" As I know common MOF resistor body is just a lacquered coating,

EN60335-1 is a safety standard for home appliance, so in which home appliance are using this PCB?

I would like to ask is this resistor considered as "Lacquered conductors of windings"? If so this is violate the safety standard.

Lacquered conductors are generally referred to the enamel coated copper/aluminium wire used for winding motors.

But anyway whether it violates safety standard or not that's a secondary question, the important point is if we consider the above described placement of resistor on a pcb trace with 220Vac(as mentioned) in the normal operation of your PCB,
and it is causing this kind of failure; then we should try to understand the root cause of failure.

Because arcing is happening due to dielectric breakdown between resistor and trace, so probably you may have to change the design as well.

 

[cyberrat]

Sounds like whoever did the PCB did not consider this in the design, if there is no choice but to put a 220v track underneath it then perhaps it would have been better off being stood off the board with looped legs, standoffs etc.

P.S. EN60335 is not just for household appliances, it covers "Safety of household and similar electrical appliances".
I.E. Commercial catering equipment, hot cupboards, Bains marie etc. amongst many others.

See https://landingpage.bsigroup.com/LandingPage/Series?UPI=BS EN 60335 for another example of what it covers.

 

[FvM]

Referring to "lacquered conductor" rules in safety standard not specifically dedicated to PCB designs seems far-fetched to me. But I think, you don't need to decide if the situation is covered by this rule.

As long as neither the solder mask nor the resistor surface coating has a voltage rating sufficient for 230 VAC, Cat. I, II or whatsoever, you need to guarantee the clearance by using spacers at the resistor terminals or supplement an insulating foil.

Creepage and clearance numbers are similar in all applicable standards, so it doesn't really matter if you refer to EN60335 or e.g. EN61010 (IEC 1010).

 

[eepty]

Thank you for all your help and sorry that I come back to this thread late. Actually in my personal opinion, this PCB design is inadequate obviously. However as I need to convince other to change the design, I cannot just say, "I believe that". Therefore I want to find some strong supporting document to support my opinion...And because of other factor I cannot do a electrical surge test on that PCB.