LEACH ® INTERNATIONAL
CORPORATION
1. HOW TO PICK A RELAY
where most misapplications begin
or
How to save $35 and lose a half-million
dollar missile
WHY THIS APPLICATIONS SERIES?
Electromagnetic relays have been making and breaking circuits for more than half a century. The scope of their applicability has been widened by advances in the techniques and technology of manufacture; industry standards have been established that increase their range of usage; and considerable effort has been expended to increase their reliability. In spite of this, operational failures have not been reduced proportionately. Why not? Because a major reason for relay failures remains the misapplication of the relay. Even with all the changes during its history, in operating principle the relay remains a simple device. It looks unprepossessing on a circuit diagram - an electrically-activated switch. Anything that easy to understand can't be hard to use, can it? This apparent simplicity is the major stumbling block encountered by relay users. The designer tends to overlook this "simple component" and does not give it the same amount of consideration given other components in the circuit. The relay starts working with a virtue turned into a disadvantage. It is simple, but not forgiving. Relay contact ratings are the most misunderstood specifications and the most common cause of relay mis use. It is not enough to call out the currents and voltages that the relay will see. Relay contacts are designed, built, rated and must be specified in terms of the specific type of application for the relay. A 5-amp, 115 volt set of contacts in one circuit may or may not be a 5 amp, 115 volt set of contacts in another application. The designer should specify the type of load, duty and environment to which the relay will be exposed. Incomplete specifications from the user often result in misapplications. The consequences can be tremendous for such a "simple" device.
THE $35 MISUNDERSTANDING
Consider a recent high reliability application for which the design engineer specified a group of 4-pole, double-throw, single phase, 5-amp relays. Several weeks after delivery of the specified units, a frantic call was received from the engineer - system check-out had just been initiated and relays were failing. What was wrong with the relays? We quickly acquired some of the failed units and examined them, discovering the symptoms of a recurring failure mode in relays - phase-to-phase and phase-to-ground arcing. (Phase-to-ground arcing pits the contacts; phase-to-phase arcing leaves the contacts looking 'blown-up' and occurs between adjacent contacts). The most prominent cause of this type of failure is the misuse of a relay with single-phase contacts to switch poly-phase power. A single-phase relay is designed with contact clearances and dielectrics that will readily handle the peak loads specified when these are imposed by single-phase voltages. In polyphase circuits, the phase angle differences between the applied voltages will result, at some time when the relay is operating, in voltages greater than the relay designed for single phase use can handle. Catastrophic failure often follows, with arcing to ground or the merging of two loads by arcing between adjacent contacts and the destruction of the contacts.
When asked, the engineer stated that he was in fact using the relays to switch a 3-phase motor to and from an ac generator. Our recommendation was that he spend an additional $35 to get a 3-phase relay designed for his type of application, but he declined on the grounds that time was short and his budget was spent. The next failures occurred during system operation. The ac power generator stopped, thrust dissipated, and the missile the relay was on fell back to earth. Total loss: one-half million dollars. Another "relay failure"?
DOUBLE-THROW CONTACTS
Double-throw contacts are a versatile arrangement that can save the user space, weight and money - sometimes! Too often these savings are only temporary, and are more than cancelled by the first failure due to misapplication.
Figure 1 illustrates a common but potentially hazardous use of a relay with double-throw contacts. Rather than switching between two power sources as shown, the relay might be reversing the direction of a 3-phase motor by swapping phases, switching one source between two loads, or just switching a load from a power source to ground. The diagram applies to all of these situations, and the potential danger of relay misapplication is high. If an arc is created as one contact is broken and is not extinguished before contact is made with the second fixed pole, the power source is shorted and heavy damage results.
All contacts have minimum current and voltage thresholds that must be exceeded before a sustained arc can be created. The minimum voltage required, a function of the material used for the contacts, is 12v for silver (typical of most metals), 15v for gold and 17.5v for platinum. A minimum current is also required, again a function of the material. For silver (typical of many metals), this is 400ma; for gold, 400ma; and for platinum, 700ma. The only lower values are those for cadmium and zinc (100ma) and for carbon (10-20ma). Both voltage and current must exceed these threshold values at the same time for hazardous arcing to exist.
If your circuit will not exceed these threshold conditions, it won't sustain an arc, then the configurations of Figure 1 can be used. If they are exceeded, you could use this circuit for selection only, and have the load carried by another set of contacts. Or consider the circuit shown in Figure 2.
The relays are both single-pole relays, each with 3 normally-open (N.O.) and one normally-closed (N.C.) contacts. The normally-closed contacts serve as safety interlocks - neither relay can be energized until the other one is de-energized.
YOUR SPECIAL REQUIREMENTS
As stated earlier, there is no single voltage/current rating that applies to a given set of contacts under all circumstances. If the specifications seem incomplete, or your requirements are unusual, call the relay manufacturer. At Leach, we have an applications group with years of product experience ready to go to work on your problem. Call and let us help.
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