Mercury Relay Advisory

The information in this section is intended to notify American Innovations (AI) customers of the manufacturer's published limitations on mercury relays.

Customers primarily purchase mercury relays for use with current interrupters. Current interrupters provide great programming flexibility to enable customers to use a wide variety of interruption cycles.

AI supplies mercury relays from a variety of respected and reliable vendors including MDI and American Electronic Components (AEC). These manufacturers publish expected relay lives of 3 to 6 million cycles depending upon the type of relay selected. AEC, the manufacturer of Durakool relays, indicates that relay life will be significantly shortened at rates exceeding 1,500 cycles per hour. MDI recommends no more than 900 cycles per hour. Depending upon the interruption cycle chosen by the customer, it is possible to exceed the manufacturer's recommendation, which would result in a likely reduction of service life.

As an example, customers who elect to use mercury relays to interrupt at 1 cycle per second will generate 3600 cycles per hour. Customers should consult the manufacturer's documentation to select the appropriate relay for the application.

Durakool information can be found on the AEC web page at the following address:

http://www.aecsensors.com/html

In the Durakool documentation, the following two situations are explained that may impact the current carrying capacity of the relay: 1) cycle rate and 2) voltage values. The sizing tables indicate a methodology for determining the maximum amperage that can be carried depending upon the cycle rate. An additional derating table for mercury relays is provided for those instances where voltage in excess of 48 V DC is being interrupted. When selecting relays, customers should be aware of both of these derating tables to choose the best relay for the application.

To avoid depolarization of the CP system, the mercury relays we sell that AI sells are of the normally closed variety. It is possible that a normally closed relay will fail at the end of its life in the open position due to a malfunction of the spring mechanism which drives the plunger/contacts into a normally closed position. If this occurs, a depolarization situation would result. AI has the following two recommendations:

  1. Customers using mercury relays should consider periodic replacement of these relays based upon engineering estimated life via a preventive maintenance program to reduce the likelihood of an open circuit failure.

  2. If fast cycle operation is common, AI recommends a solid state relay for maximum life and reliability.

Although most interrupters are capable of being programmed for faster interruption cycles, AI recommends that customers not exceed 900 cycles per hour (or a cycle time of less than four seconds) and will not warranty devices that are utilized in this manner. AI adds a serial number to relays that is supplied that we supply to enable the tracking of the shipment date. The approximate age of the relay may be used to assist AI in return material response, failure analysis, and warranty claims. Manufacturers will be consulted to help determine failure cause and to advise customers that warranties may not be honored if it is deemed that the mercury relay is used outside of recommended parameters.

If you have any questions or need further assistance, give us a call at 1-800-229-3404 or email us at support@aiworldwide.com.