Response to Ray Russell at Leco (I originallly responsed to Ray directly but got an e-mail rejection notice) -
Yes, mercury is a " No No" nearly everywhere now. You have a common problem - particularly compared to large mercury switches. The "noise" that is causing you problems may be from the contact arc or the collapsing field of the relay coil itself. If the coil is DC then a diode can usually effectively suppress it - sometimes at the cost of reduced relay contact life. If AC it is a little harder and requires an R/C network or bipolar device like MOVs or "transorbs". Applying an R/C or MOV across the load (or occasionally across the contacts) can often suppress the load switching noise. If across the contact, R/C networks have a similar leakage problem to an SSR so you may not be able to do that if hard contact isolation is required. You may be OK with an SSR for most applications unless the relay contact is the only isolation protecting the service personnel who might work on the load (in which case the relay probably needs 2.8 or 3mm contact gap also depending upon applicable standards). It might be worth confirming. An SSR with zero-crossing turn-on would be ideal from a noise viewpoint and they are readily available. Non-zero triggered devices (called random) may make the noise problems worse since they can switch on anywhere in the cycle and generate a large current spike if they switch appreciable above zero - especially on a cold heating element. The nature of SSRs usually makes either type turn off near zero current; which is also near zero voltage on non-inductive heaters where phase shift is minimal. SSRs do have small leakage currents from the triacs or SCRs inside as well as from an internal "snubber network" usually included to prevent false switching and/or device damage. They can only be used if there is a hard contact primary power disconnection of sufficient contact gap to protect service personnel. I'm not sure how familiar you are with SSRs, but they also have very specific requirements for heat-sinking to control device junction temperature. Heatsinks can be physically large and costly depending upon your environment. As a supplier of both electromechanical relays and SSRs, we can assist you if needed. Best regards, Harold Leipold Siemens Electromechanical Components Inc. Princeton, IN Tel 812-386-2161 Fax 812-386-2616 E-mail [email protected] ======================== [email protected] on 12/16/97 12:35:45 PM Please respond to [email protected] To: [email protected] cc: (bcc: Harold Leipold/PRI/SEC/US) Subject: Mercury Switch Replacement Greetings, Some time ago, I raised the question about Mercury switches to the group. It was determined that Scandinavia would not allow Mercury type switches. We went with a relay recommended as an alternate by the mercury switch vendor. Since then we have had problems with contact arcing (it is switching 40A at 230V) and spikes getting back on the 24V control line. Diodes and surge suppressers have been installed. And seem to have fixed the problem for now. (This is also a relay used to shut off the power to a heater circuit, when the coolant is low, or a some other fault occurs. So I believe a open contact is required over a solid state relay). My question is, has anyone seen a good replacement for a Mercury displacement relay for switching high currents? It appears that the design of the MDR is superior over dry contact type relays. I've checked with most of the big relay manufacturers and have not found a good solution. Thank you, and I hope that you all have a Happy Holiday Season, Ray Russell Leco Corporation 3000 Lakeview Ave. St. Joseph, MI 49085-2396 Ph (616) 983-0308 Fx (616) 982-8964 email: [email protected]
