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Has anyone used these? #7887 02/27/02 11:16 PM
Joined: Feb 2002
Posts: 33
JohnnyB Offline OP
I'm looking for a way to switch 25-30 240vac resistive amps with no sound. Has anyone used these mercury displacement relays?
They look like just a heater and some merc, but the contacts are still there. I'm also looking at solid state, but they are high$$$$. I haven't found one to switch off of 24vac input. The purpose is to switch 5KW heaters inside a residential space where sound is a issue.

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Re: Has anyone used these? #7888 02/28/02 01:18 AM
Joined: Oct 2001
Posts: 135
wolfdog Offline
Used mercury relays for electric heat in a broadcast studio. They worked great and were silent.

Re: Has anyone used these? #7889 02/28/02 11:44 AM
Joined: Jul 2001
Posts: 599
JBD Offline
I just used a solid state unit from Carlo-Gavazzi (#RN1A23A25E) for $56.00. It is a 1 pole device rated for 25A @ 230VAC, the control voltage is 24-230VAC/DC. As long as you provide a 2 pole disconnect and OCPD you can use a 1 pole device for the temperature control.

A problem with solid state devices is the lack of a true off position like you would get with the mercury relays.

Re: Has anyone used these? #7890 03/02/02 05:17 PM
Joined: Dec 2000
Posts: 4,283
electure Offline
Mercury (Hg),
Used to be used in all of the "silent switches".
They are a self cleaning, non arcing contact, and work great.
Good luck trying to install them without some environmental troubles

Re: Has anyone used these? #7891 03/02/02 05:42 PM
Joined: Oct 2001
Posts: 135
wolfdog Offline

Re: Has anyone used these? #7892 03/02/02 08:18 PM
Joined: Sep 2001
Posts: 806
NJwirenut Offline
I would also recommend the mercury displacement contactor over solid-state, especially if the switch loop operating the coil circuit is of any appreciable length.

I did an installation where a bank of halogen lights was fed from 208y/120, through a 3 phase Crydom SSR. The control input rating on the relay was 24-120 VAC, and I drove it with 120 VAC, developed from a small 208/120 control transformer in the relay enclosure. The "switch loop" controlling the relay was ~ 200 feet long, consisting of 2 wires in 1/2" EMT, to a relay in a control cabinet at the far end of the run.

A few days after the installation, I received a callback stating that the lights could not be completely shut off without opening the disconnect ahead of the SSR. The lights would flicker on and off at low brightness at random intervals. A bit of troubleshooting revealed that there was a low voltage (~15 V) present across the "coil" terminals of the SSR, even with the control circuit open. Capacitive coupling between the 2 parallel wires in the switch loop, coupled with the low (milliamps) current requirement of the SSR was allowing enough potential to develop across the input of the SSR to partially energize the load. Placing a small 120 VAC incandescent bulb across the SSR "coil" provided a sufficient load to stop the problem temporarily, but a mercury displacement contactor (with a real magnetic coil) was installed as a permanent repair.

Re: Has anyone used these? #7893 03/03/02 02:10 AM
Joined: Feb 2002
Posts: 33
JohnnyB Offline OP
Hey NJwirenut,

Wow,.....interesting. That was a capacitive reaction between the two wires? Capacitive and not inductive? The length of wire was like the coils of conductor inside a capacitor and 200' was enough to make 15v across the input terminals of the SSR, right?
Did you know what was going on right away, when you measured the 15vac?

Would using twisted shielded wire prevent stuff like that or not in this application?

Re: Has anyone used these? #7894 03/03/02 12:21 PM
Joined: Sep 2001
Posts: 806
NJwirenut Offline
I would assume capacitive, as there was no appreciable current flowing in the wires, to make the wires act like windings of a transformer.

The problem was pretty obvious when the voltage was found across the SSR coil with a high impedance DMM, but dropped to zero when a test light was bridged across the coil. The "coil" of an SSR is actually an LED, which only requires 10 mA or so to energize the relay.

Shielded wire probably would have helped, but only if the 2 wires were individually shielded from each other, and the shields were grounded. 2 wires with an overall shield (the more common configuration) would still be subject to capacitive coupling. The setup I used (2 wires in metal conduit) is equivalent to a 2-wire shielded cable. Reduces interference/coupling from outside sources, but not between the 2 conductors.

Re: Has anyone used these? #7895 03/03/02 09:41 PM
Joined: Nov 2001
Posts: 273
circuit man Offline
sounds like the mercury relays listed would be the way to go. we use them in our electric heated washers.silent mostly, & pretty darn well leak proof.


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