How (not) to destroy a relay

A relay is generally characterized by its rated coil voltage for operating the contacts, and the maximum switching voltage and current that the contacts are designed for and guaranteed by the manufacturer (in terms of the minimum number of switching cycles).

If the relay is already built into a device, such as a programmable control unit, we are mainly interested in the maximum switching voltage and current of the outputs (contacts). However, there is usually a mention that these values are given for a resistive load. This is a well-intended warning from the manufacturer to prevent contact destruction.

Contact load

The effects occurring at a relay contact depend greatly on the size and type of the load, the current, the contact size and material, the operate time and the contact bounce. 
For DC loads, the maximum switching current is usually lower than for AC loads. While AC current periodically drops to zero, DC current does not; therefore, in the DC case, any electrical arc ignited when the contacts are pulled apart is very difficult to suppress. The arc discharge lasts longer in a DC circuit compared to an AC circuit. Hence it is important to distinguish in the datasheet between the maximum switching load specifications for DC loads and for AC loads.

In addition to the rated load voltage and current (i.e. steady-state values), the relay lifetime is also greatly influenced by the inrush current and switch-off spikes that occur when the load is connected/switched on or disconnected/switched off. For capacitive and inductive loads, these can be orders of magnitude higher than the rated values, and an undersized relay or a lack of the appropriate limiting circuitry can easily result in destroyed contacts.   

Switching a resistive load

When a purely resistive load is connected to the relay output, the entire allowed range of voltage and current can be used without any problems. A resistive load is defined as a device with its inrush and switch-off currents equal to the steady-state values. That is, a load whose electrical resistance is always the same and does not change in time; therefore, the current is also constant from the switch-on moment to the switch-off moment.

Usually, only resistors – purely resistive components designed for this particular purpose – exhibit this behavior. However, even devices that do not exhibit a constant resistance can be regarded as resistive loads, provided that their resistance only changes slowly with time.

Devices that mostly behave as resistive loads include:

• Incandescent bulbs
• Various LED lights
• Heating elements
• Voltage inputs of measuring instruments (multimeters, oscilloscopes, PLCs etc.)

The above examples of resistive loads do not necessarily exhibit a truly constant electrical resistance (it may change somewhat over time); however, if the relay contacts are appropriately oversized (approximately 2x to 4x), they do not usually cause any problems or contact damage.

Switching a capacitive load

 In contrast to a resistive load, the situation is entirely different with a capacitive load. When a capacitive load is connected to a power source, it starts to draw a very large current that quickly decreases. Therefore, when a capacitive load is switched on, there is a large inrush current that can significantly exceed the maximum allowed switching current of the relay contacts. When the capacity of the load is completely discharged, this inrush current is only limited by the parasitic resistance between the contact and the capacitive load. As the load charges, the current decreases.

For the relay, the switch-on moment is the biggest problem: the contacts carry the highest current and can be even welded together if this current exceeds the allowed maximum even for a short time. Remember that in a circuit with a capacitor, the inrush current can be 20x to 40x higher than the steady-state current.

The significance of inrush currents can be demonstrated by the “Inrush current” values in the technical specifications of switching power supplies or other appliances. Even with a small switching power supply or power converter (e.g. with an output power around 30W), the inrush current can reach tens of amps (32A is no exception). This kind of current can be drawn by a completely discharged (that is, disconnected for a long time) power supply for up to 100ms after being powered on. Since the current quickly drops to the rated value afterwards, the inrush current is not significant from the long-term perspective. However, its effect can be observed when a switching power supply (or a device containing one) is connected to an undersized circuit breaker: when the device is turned on, the circuit breaker may often trip. It is also common to observe the inrush current effects as sparks (sometimes quite prominent) when a switching power supply that has been disconnected for a long time is plugged in to an electrical outlet.

Ideally, the maximum switching current of the relay contacts (or any switches in general) should be close to the specified inrush current, or the inrush current should be limited.

Examples of devices that behave as a capacitive load:

• Switching power supplies – the higher the power output, the higher is usually the inrush current
• Long transmission lines or cables – due to the parasitic capacities between the wires in cables longer than about 10m
• Various voltage filters – e.g. loudspeaker crossovers
• The inrush current of capacitive loads can be reduced by connecting various elements in series with the load:

Connecting a resistor / NTC thermistor: the inrush current is reduced thanks to the voltage drop at this element (the voltage at the capacitive load increases gradually, not suddenly). However, there are additional losses in the steady state.
Connecting a choke (inductor): the inrush current is compensated (reduced) and, at the same time, there are no big steady-state losses (the choke exhibits only a small wire resistance).

Switching an inductive load

The biggest “enemy” of a common relay is an inductive load, such as a solenoid or an electromagnet. Its behavior is the most damaging, capable of completely destroying (welding or burning) the relay contacts. It behaves in the opposite way compared to a capacitive load. The switch-on effects are not a problem; the current increases gradually up to the rated current specified for the component (during this time, the inductive load behaves as a choke).

However, the problems come when the relay contacts open and the inductive load is disconnected. Due to the underlying physics principle, the inductive load tends to maintain the same current that was flowing through it before the disconnection. To this end, a voltage of the magnitude and polarity necessary to maintain this current is temporarily induced at the terminals of the inductive load.

Therefore, when an inductive load is switched off, there is a voltage spike that can damage the load itself (winding insulation) as well as the contacts of the disconnecting relay. The voltage spike is influenced by the inductance (the higher the inductance, the higher the voltage spike) as well as the switch-off time and method. The switch-off spike is the highest when a device is quickly electronically disconnected (snap-action) and there is no spike-limiting circuitry. The effect is especially adverse in case of high inductances, higher supply voltages and higher duty cycles (the ratio of electromagnet on-time to the cycle period). In practice, voltages of up to 30x the rated supply voltage (or switched voltage) are common.

When the inductive load is snap-action disconnected by opening relay contacts, the lack of limiting circuitry can lead to generated voltages of hundreds of volts, able to ignite an electric arc between the contacts. This is a “reliable” way to damage or even permanently destroy the contacts.

Examples of devices that behave as inductive loads:

• Solenoids / electromagnetic valves
• Electromagnets
• Electromotors
• Transformers
• Fans
• Relay coils

Surges can be limited by connecting various electrical components in parallel to the inductive load in order to shunt the switch-off spike voltage generated at the inductive load (coil) terminals:

• A diode and a resistor (for DC circuits) – the diode as well as the resistor need to be sized according to the switching frequency, spike voltage and the load input power. The diode breakdown voltage must be higher than the circuit supply voltage, and the maximum forward current must be higher than the current through the load (inductor) when switched on.
• A “spark quenching capacitor” (for AC circuits) – a RC circuit connected in parallel to the contact, where R = RL a C = L/RL2
• A circuit with two Zener diodes or a transil – the Zener diodes or the transil clamp the spike voltage to the specified threshold voltage. A disadvantage is that in case of overload, they fail and remain permanently open, shunting the load.
• A varistor circuit – a voltage-dependent resistor connected in parallel to the load and satisfying the condition 2 < Umax/Uj < 4 (where Umax is the varistor voltage, Uj is the supply voltage).

Various ways to protect relay contacts from the effects of switching an inductive load – from left to right: a diode, a spark quench capacitor, Zener diodes or a transil, a varistor.

Conclusion

The conclusion is that a switched load does not always follow the rated current and voltage specifications. These are only valid for resistive loads / appliances, which, in practice, are the exception rather than the norm. Therefore, the relay (or other) contacts should be always adequately oversized (with respect to the switched voltage and expected current), and the load should be equipped with additional circuitry depending on its behavior – that is, whether the load is generally capacitive (filters, switching power supplies) or inductive (solenoids and electromagnets). For some devices, the specifications indicate the maximum contact load as “Maximum switching current 16 (4) A, 250 V~”. In this case, 16A is valid for a resistive load, 4A for an inductive load.

If the capacitive or inductive load lacks the appropriate limiting circuitry, it can easily damage or destroy the relay as well as the control device where the relay is integrated – sensor, PLC, remotely controlled unit, etc.