Using SSR's in a 3-phase environment

[adavison17]

G'day all,

I'm putting together a distribution box that takes 3 x 20A power curcuits in (each on a different phase), and switches these to separate 10A loads (two per phase). The loads are 2400W heating elements in a small brewery.

I'm not familiar with SSR's, and have the following question:

If I am attaching the SSR's to a heatsink, do I need to keep the SSRs on each phase electrically isolated, or can all 6 SSRs sit on a single large heatsink?

I thought I had the answer when I saw a text that indicated that the heatsinks need to be connected to a protective earth, and figured that it would be sufficient to connect them all to the common earth of the 3 phase supply.

However on measuring the potential between neutral and the SSR back I noticed it was 3-5V, leading me to believe that connecting all the SSRs electrically is probably not wise.

Furthermore, the whole device is in an aluminum rack case, so I'll have to build some insulating stand-offs for the heatsinks...

Any advice would be very much appreciated.

Regards,

Andy

 

[crutschow]

You are probably just seeing voltage from capacitive coupling to the SSR back plate. You can check the isolation by measuring resistance between each of the power leads to the SSR back (unpowered of course). It should measure >10MΩ if the back is properly isolated.

 

[adavison17]

Of course - thank you Carl. Indeed it comes up as an open circuit between the back plate and the terminals.

Regards,

Andrew