Understand Relay Specifications to Get the Most Out of Your Switching System

    Relay specifications aren't simply numbers on a data sheet-you need to take them seriously. Operating a relay used outside of its specifications can severely shorten its life and cause switching system failures and even potentially damage the UUT (Unit Under Test). With that in mind, let's look at some common relay specifications and the impact they have on switching systems. 

    Life Expectancy 

    Relays have moving parts and operating them causes wear and stress that will eventually lead to relay failure. The life expectancy specification provides information on when you can expect relays to mechanically wear out. Essentially, this specification is the number of times a relay can operate under no load or light load conditions where contact wear, relay temperature and forces acting on the moving parts are simply the result of mechanical activation. 

    There are two types of mechanical relays: reed relays and electromechanical relays (EMRs). In general, instrument grade reed relays have the longest mechanical life because the relay has few moving parts. The reed relay blade bends rather than being moved on a pivot point, and the contact is contained in a hermetically sealed glass envelope, so it is less susceptible to contaminants and mechanical defects. 

    EMRs tend to have a shorter mechanical life than reed relays, but they have greater power handling capacity. 

    Maximum Switching Volts 

    The maximum switching voltage of a relay is the maximum voltage that can be across the contacts whether the relay is open or closed. Operating a relay with high voltages present can cause arcing, and this in turn erodes the contacts and eventually degrades contact performance. For more information on this topic, see our Knowledgebase article, Hot Switching Relays. In a switching system, voltage ratings may be limited by factors such as the spacing between traces on the PCB or connectors. When Pickering designs a switching module, we use the voltage rating of the relays on the Example specifications from our high-density PXI matrixboard to determine the minimum acceptable spacing between circuit board traces. 

    If both positive and negative voltages are present, the difference of these two voltages must be considered. For example, if your switching system will be switching three-phase power supplies, the voltage across a relay will be greater than the individual voltages of each phase. 

    For Pickering Interfaces' switching systems, unless otherwise stated, voltage specifications refer to voltage differential. For example, if a switch module is rated at 150V, then it can be used for switching signals in the range 0 to 150V, -150V to 0V, or -75V to +75V. 

    It is also important to remember that the maximum switching voltage of a switching system may be less than the maximum switching voltage of the relays because relay specifications are usually defined using resistive loads. Because switching systems have some amount of capacitance (the largest contributor to this is the capacitance between traces on the PCB), the maximum switching voltage system specification may be lower than the relay specification. 

    Cold Switching Voltage 

    Relays may be able to sustain higher voltages across their contacts than the maximum switching voltage, provided no attempt is made to operate the relay while the signal is applied. This specification is called the cold switching voltage or standoff voltage. 

    Relays with high standoff voltages can be useful in insulation testing, but the user MUST avoid switching the relay while the voltage is applied since it exceeds the contact voltage rating when being operated. 

    When a switching system has a cold switching voltage specification, this means that the spacing between the PCB traces have been designed to withstand this voltage. 

    Switch Current 

    When a relay is hot switched, the switch current is the maximum current that the relay can sustain when being opened or closed and not sustain contact damage. 

    Carry Current 

    If a relay's contacts are already closed, the relay may be able to sustain a higher current than the switch current. This is called the carry current. The carry current is normally limited by contact resistance, which causes the contacts to heat up. When a relay is carrying a current greater than the switch current, the relay must not be opened until the current is reduced. 

    Pulsed Carry Current 

    Some relays or switching systems may have a pulsed carry current specification. A pulsed carry current simply heats the relay contacts, it does not create the same arcs that hot switching creates. 

    Relay contacts have sufficient thermal mass that this pulsed current does not cause the contacts to overheat, and as a result, does not damage the contacts. The pulsed current could be a single event or could be repetitive, but if it is repetitive, then some care is needed to make sure the net effect does not create a thermal problem. A typical 2A EMR might, for example, sustain 6A for 200µs. 

    Also remember that thermal effects are proportional to the square of the current (assuming a constant contact resistance). So, if the pulsed carry current is three times the carry current, then the duty cycle of that signal must be 10% or less. This is particularly true if the contacts are required to carry current between the high current pulses. 

    Power Rating 

    Some users ignore the power rating, but this specification has a major impact on relay life. A signal at both the maximum switching voltage and the maximum switch current will generally exceed the power rating of the relay. 

    For example, a relay with a 60W power rating, may have a maximum switching voltage of 250V and a maximum switch current of 2A. A 250V, 2A signal has a power of 500W, which exceeds the power rating of the relay by nearly an order of magnitude. To stay within the power rating, a signal with a maximum voltage of 250V, should have a current of no more than 240mA. 

    Most relays, therefore, have a complex useful working area. The higher the switched voltage, the lower the maximum switch current must be for a relay to handle it safely. 

    In addition, at high DC voltages, the power rating for a mechanical relay is lower than the rating at lower voltages because the closing or opening of the relay creates an arc that in turn creates a plasma which can damage the contacts and relay materials. Users should always check the load curves provided on a relay's data sheet when DC signals are being switched above 30V.

    For the same reason, frequent operation of a relay under high loads can degrade the life as the arc raises the temperature inside the relay. 

    The power rating specification applies to signals that are hot switched, but power ratings are often different between DC and AC loads. See our Knowledgebase article, Hot Switching Relays, for more information. 

    There is rarely a power rating issue when using solid state relays that have been designed for fast on/off operation because they can often sustain both high voltage and high current without damage. The fast on/off times ensure that the dissipation during state transition is low, and there are is no arcing when using a solid-state relay. 

    Minimum Switching Voltage 

    Some types of relay have a minimum switching voltage that must be present for the relay to switch reliably. This is especially true for relays used to hot switch signals where contact wear can occur and expose the underlying materials. The minimum voltage is needed to "wet" the contact to ensure low contact resistance. 

    Reed relays are particularly effective for low voltage switching because their contacts are hermetically sealed in glass, and contaminating films cannot build up on the contacts. Some relays designed for telecommunications applications also have minimum voltage ratings because they have gold contacts. High power relays often require higher minimum voltages than low power relays once the protective gold flash has been eroded either by hot switching or mechanical wear of their high-pressure contacts. 

    Operate Time 

    The operate time specification can sometimes be confusing to users, but can be critical in precise timing situations. An application not taking into account relay closure times may mean that a particular measurement may not be captured correctly because the relay was not yet closed and carrying the signal. 

    For a switching system, the operate time specified on the data sheet includes the time that the software takes to process a driver instruction as well as the time the relay takes to operate and settle. The drivers used by Pickering Interfaces include information on relay timing, and the driver will prevent access to the switching system until completion of the settling time unless the driver wait state has been overridden. Some switching systems may require more than one operation to complete a state change for the system in order to ensure there are no accidental make before break operations. More information on this topic can be found in our Knowledgebase article on Resistor Card Transient Values.

    Module Switching Specifications vs. Relay Switching Specifications 

    Relay specifications do not always apply at the module level for a variety of reasons. The PCB design of the module, for example, can affect both the maximum voltage and maximum current specifications. Spacing between the PCB traces will affect the voltage specification, while trace width will dictate the current specification. What this means is that modules that have more relays generally have lower maximum voltage and current specifications. 

    The reason for this is that denser modules have PCBs with thinner traces and closer spacing. In addition, some modules have significant capacitance. This capacitance can lower the hot switch voltage and current ratings since it can create an inrush current on contact closure. The larger the switching system, the more likely this is to happen. Using long cables in your switching system will also likely have an impact on the ratings. This is due to cable capacitance (which can be broadly assumed at 100pF per meter for many applications). 

    Data sheets for Pickering Interfaces products specify module level performance and generally apply to systems with resistive loads, since these are the only unambiguous working conditions that can be stated. 

    For other types of loads, you'll have to reduce these ratings.

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