[Amps] Solid state relays..again

[Jim Thomson]

When I had my old drake T4XC transmitter, about every 40th time I turned it on.... it just went
click and turned on. The other 39 times it made a helluva thump noise inside the xfmr!   There was
NO step start on the outboard AC4  power supply.  My conclusion was I hit a zero V crossing  aprx
every 40th time.   The zero V cross  SSRs  caught my eye soon after that. 

The SSR is just an on-off switch.......and so is a contactor.    On a lot of contactors, the contacts are field
replaceable and ditto with the solenoid coil.  Unlike a relay, a contactor is essentially two sets of contacts
in series...per pole.   Any arc is split into 2 x arcs  = zero arcs.   They open off fast due to the typ huge spring tension. 
Of course you need one helluva lot of magnetism to overcome the spring tension.  Small stuff like P+B 40A  relays
have next to nothing for spring tension, and contacts are prone to welding shut.  

I find it amusing that all our HVAC stuff at work still uses contactors.  Sure  P+B showed an example of a 150 hp electric motor
run on 600V 3 phase..all using a huge mess of SSRs in series parallel.   Problem is.... it would be a trick to trouble shoot 
and isolate any int problems...or hard faults.

Jim  VE7RF