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I am working on a design that consists of a mains power entry module which is a filtered, fused mains switch. This switches the live and neutral conductors to feed 4 switch mode power supplies. Each power supply specifies a cold start inrush current of < 40A at a max input voltage of 264Vac input (UK here). The total load on the mains supply will never exceed 300VA giving a max load current of just over 1A.

I am experiencing some bad arcs across the switch contacts when turning the unit on and off due to the combined inrush of the 4 power supplies. I am new to the subject of contact suppression but I do know the switch will not last without any consideration. I did some reading in to possible solutions and fit an RC snubber (100R 0.1uF 250Vac) across both live and neutral switch contacts. This seemed to solve the issue and the switch no longer arced, however after a lot of persistent use due to testing (~150 on-off cycles in a number of days) the switching arcs have returned even with the suppressors still fitted. I checked for any failure by simply measuring the resistance but that only eliminated the chance of a short circuit failure of the snubber, I'm not sure why they are no longer supressing the contacts.

I looked in to other possible solutions, such as series NTC thermistors, varistors, TVS diodes but am not sure what is best. I think the RC snubber is a good solution but I am not sure how to best calculate the values for R and C that best suit this design. I also have to consider the implications if the RC network fails, resulting in a potential short bypassing the mains switch.

My questions are - should I be trying to limit the inrush current, or focus on suppressing transients caused by the switch contacts? Also, what is the best solution for my design?

Parts link/ref -

Mains switch - https://www.schurter.com/cns/datasheet/CD21.1501.151

SMPSU 1 (2 of) - http://powersolve.co.uk/sites/default/files/MHP40-100W%20Series.pdf

SMPSU 2 (2 of) - https://www.us.tdk-lambda.com/ftp/Specs/zms100.pdf

RC snubber - Farnell Part# 1438457 Manuf Part# FE-SP-HDR23-100/100

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  • \$\begingroup\$ Has the capacitor failed? Check by replacing it with a new one. If it has failed you may need to get a higher rated capacitor (400Vac) and it must be Y-rated for safety. \$\endgroup\$ – Steve G Oct 4 '17 at 10:52
  • \$\begingroup\$ It's an RC network that's encased in plastic, so I'll need to replace both of the snubbers to check, and yes maybe i'll have to try my own RC network with higher rated components. I was surprised the snubber was only rated at 250Vac because if mains fluctuates even by 10% I'll be stressing the components, let alone from the transients when the contacts switch. \$\endgroup\$ – Nick B Oct 4 '17 at 11:34
  • \$\begingroup\$ Looking at X/Y capacitors, how can a capacitor be both X1 & Y1? such as this: uk.rs-online.com/web/p/ceramic-single-layer-capacitors/8848709 \$\endgroup\$ – Nick B Oct 4 '17 at 15:53

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