Hi Steve:

 

Mains circuits are subject to lightning and switching transients both 
line-to-neutral and line/neutral-to-ground.  The transients are normal; to 
prevent insulation breakdown and consequential electric shock, the electric 
strength of mains-to-ground and mains-to-other circuits insulations must exceed 
the expected transient voltages.  (Electric strength requirements based on 
transient voltages are specified in IEC 60664-1, supported by research by 
Stimper.) 

 

The mains transients can be capacitively coupled to secondary circuits, 
depending on the configuration of the isolation scheme.  In most cases, the 
isolation scheme and the impedance of the secondary circuit attenuates the 
transient voltage to near zero. This point seems to be recognized by IEC 
60101-1.

 

Then, as Brian Kunde has said, secondary circuit devices such as relays, 
solenoids, and switching can generate transients in the secondary circuits.  In 
most cases, such transient voltages are small and are attenuated by the 
secondary source impedance. 

 

IEC 60601-1 has a similar requirement, although it includes clearance 
requirements for no-transient secondary circuits.  IEC 62368-1 does not have 
such a requirement.  I wonder what evidence or research supports transient 
voltages in secondary circuits?  

 

Best wishes for the holiday season,

Rich

 

From: Steve Brody <sgbr...@comcast.net> 
Sent: Thursday, December 1, 2022 12:51 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Secondary creepage/clearance

 

I have a client who has a secondary pwb that has traces and vias that may have 
100 vdc on them adjacent to ground.  

 

Per 61010-1 there is a requirement for spacing and/or dielectric test, both 
depending on what the mains voltage is.  

 

The question is why is the mains voltage a consideration or concern if the 
100vdc secondary voltage is several layers of impedance and circuitry from the 
mains?  

 

Is it a concern that a surge on the mains would trickle down to the secondary 
circuit, or is there another reason/rationale?  

 

I suggested that a dielectric test per Table 6 [in A1] would suffice and put 
the issue to rest for this product, but the question from the designers remains 
as to why is it a concern in the standard of what the mains voltage is. 

 

Is there anything in the standard, that I haven't found, that does not require 
Table 6 to be followed if there is no way for a mains surge to impact the 
secondary voltage?  

 

I look to the experts for an explanation.  

 

Thanks, 

 

Steve Brody 

 <mailto:sgbr...@comcast.net> sgbr...@comcast.net 

C - 603 617 9116 

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