Switching and Relay Ratings

Relay ratings are usually presented as simple numbers on a data sheet, but are often prone to misunderstanding. Exceeding a relay rating can severely shorten the relay life. So here is a handy guide to the common terms used in describing a relay and the switching systems they are used to create.

Mechanical Life

Relays are constructed from moving parts, and operating them does cause wear and stress that will eventually lead to relay failure. The Mechanical Life provides information on the first failure point that is caused by these mechanical processes. Essentially it is describing the number of operations a relay can sustain under no load or light load conditions where contact wear, relay temperature and forces acting on the moving part are simply the result of mechanical activation.
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 an hermetically sealed glass envelope so is less susceptible to contaminants and mechanical defects.

EMR's tend to have a lower mechanical life but greater signal handling capacity.

Voltage Rating

This refers to the maximum voltage that can be across the contact when the relay is opened or closed. Operating the relay with high voltages present can cause arcing and this in turn erodes the contacts and eventually degrades the contact performance. Find more information on hot switching relays >>.

In a switching system it should also be remembered that voltage ratings may be limited by other factors such as track spacing on the PCB or connectors. If both positive and negative voltages are present it can be the difference between these two values that needs to be considered. Examples of this include where 3 phase power supplies are being switched, the difference in voltage is greater than the individual voltages of each phase.

For Pickering Interfaces switching solutions unless otherwise stated voltage ratings for the switch system refer to the weakest part of the design as far as voltage ratings are concerned. So if for example 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. The voltage rating of the relay is used is used to determine the minimum acceptable track gap on the switch design.

It is also important to remember that in a switching system the voltage rating of the system can be less than the relays because relay ratings are usually defined into resistive loads. It is common for relay systems to have their own self capacitance (dominated by track capacitance) that may lower the voltage (or power) the relays can handle.

Cold Switch Voltage Rating

Relays may be able to sustain higher voltages across their contacts than the Voltage Rating provided no attempt is made to operate the relay. Relays with high stand off 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.

Where the switching system has a cold switch rating then track gaps in the PCB design are designed to the cold switch rating. Some users also refer to cold switch ratings as the Standoff Voltage.

Switching Current

When a relay is hot switched there is a maximum current rating which the relay can sustain when being opened or closed, this is referred to as the Switching Current.

Carry Current

If relay contacts are already closed the relay may be able to sustain a higher current than the Switching Current. The carry current is normally limited by factors such as contact resistance causing heat build up in the contact area. Where a relay is carrying a Carry Current greater than the Switching Current the relay must not be opened (or closed) until the current level is reduced.

Pulsed Carry Current

Some products include a pulsed carry current rating. In applications where the switching system is cold switched then once the contacts are closed a pulsed current simply heats the relay contacts - it does not create the same arcs as occur when hot switching.

The relay contacts have sufficient thermal mass that this pulsed current does not cause the contacts to heat unduly and so 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 sustain for example 6A for 200us. Also remember that thermal effects are proportional to the square of the current (assuming a constant contact resistance), so increasing the current by a factor of three needs the duty cycle to be around 10% or less., particularly if the contacts are required to carry current in the times between the current pulses.

Power Rating

Some users ignore the issue of power rating, but power rating has a major impact on relay life. A relay (for example) rated at 250V and 2A is unlikely to have a long life if it switches a signal with maximum voltage and current present (in this example 500W). The relay vendor will specify a maximum power (for example 60W) - so if the relay is carrying 2A on closure then the voltage just before closure must be less than 30V in this example. This causes the relay to have a complex useful working area - the higher the switched voltage then the lower the maximum switching current for a relay. It should be noted that at high DC voltages the power that can be handled by a relay often declines 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 more readily. That can make the ratings curve not as simple as being power limiting curves, users should always check the load curves that are present on the data sheet when DC is 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 use of a power rating always implies that a relay is hot switched. Power ratings are often different between DC and AC loads. See Hot Switching.

For EMR's it is unsafe to assume that maximum power rating applies at maximum DC voltage, users should always refer to the load graphs supplied for further guidance. Typical EMR's have a lower power rating at high voltage and the only satisfactory way identifying if the conditions are within specification is to carefully look at the supplied load data.

For solid state relays which have been designed for fast on/off operation there is rarely a power rating issue since 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 no arc generation issues.

Minimum Switching Voltage

Some types of relay have a minimum voltage that must be present to reliably switch, especially if they are used in hot switch environments where contact wear can occur and exposes 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 since the contact area is hermetically sealed in glass, so avoiding contamination films building up. Some relays designed for Telecoms applications also have good minimum voltage ratings based on the use of gold contacts, high power relays often require rather higher minimum voltages once the protective gold flash has been eroded either by hot switching or mechanical wear of their high pressure contacts.

It is also quite common to see time varying resistance on closed contacts - the resistance reducing with time - for higher capacity relays.

Operate Time

Operate times can be confusing to users. In a switching system the operating time referenced on the data sheet refers to the time that the software takes to process a driver instruction plus the time the relay takes to operate and settle.The drivers used by Pickering Interfaces include information in the driver on the 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.

Module Switching Ratings versus Relay Ratings

Relay ratings do not always apply at the module level for a variety of reasons. Some aspects are controlled by the design rules used during layout, for example track clearances might limit the voltage ratings of a module, track widths might limit current capabilities. There are always compromises in product design between density and ratings. In addition some modules can introduce significant capacitance and this might impact the hot switch ratings since it can create an inrush current on contact closure, the larger the switching system the more likely this is to happen, the longer the cables in a system the more likely they are to impact the ratings dues to cable capacitance (which can be broadly assumed at 100pF per meter for many applications).

Data sheets for Pickering Interfaces products refer to the module level performance and generally reference performance into resistive loads since these are the only unambiguous working conditions that can be stated.

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