Compatible Loads

When designing an installation, it is important to consider the characteristics of the loads being controlled by the relay, as well as the intended duty (the number of operations per day and the interval between operations) when deciding how much load should be connected to a single relay channel.

The contact material of the relay component used in each channel of the relay units is chosen to be optimal for handling the high peak inrush currents commonly encountered when switching modern AC lighting loads such as Self Ballasted Compact Fluorescent or LED lamps ("SBL" loads) and LED drivers.

It may also be used for other traditional lighting load types such as Incandescent and Fluorescent Lamps, Extra Low Voltage lighting transformers (electronic or iron core), and small motors such as exhaust fans.

The different types of loads have different characteristics when switched, which can have a significant impact on the life of the relay contacts.

Resistive Loads

Loads such as heating elements have a predominantly resistive characteristic. Resistive loads do not present any adverse conditions when switching either on or off.

NOTE: The wirewound heating elements may have a small amount of inductance but this is generally not a significant concern when switching.

Incandescent Lamps

Incandescent filament lamps are predominantly resistive, but have a significantly lower resistance when cold. When switching on the filament reaches its working temperature within several mains cycles, however during this time the current may be up to 10 times the running current, which causes additional stress on the relay contacts. The incandescent rating of the relay is based on the running current of the load and is therefore lower than the resistive load rating.

Capacitive/Electronic Loads

Loads with electronic convertors such as LED downlights, LED drivers, and Fluorescent Luminaires have a capacitance at their input, which must be charged up to its working voltage when the load is switched on.

This results in a high inrush current at switch on. Fluorescent lighting with iron-core ballasts and PFC capacitors have a lower inrush of a longer duration compared to more modern LED lighting convertors which can have an extremely high but relatively shorter inrush current.

This is not only more stressful on the relay contacts, but is often also a problem for the magnetic trip function in For this reason the rating for LED lamps (the "SBL" rating) is significantly lower than the rating for Fluorescent Lamps (the "AX" rating).

Inductive Loads

Loads with wound components on a magnetic core such as contactor coils, neon lamps with iron core ballasts, iron core ELV lighting transformers, and motors have an inductive characteristic. An inductive characteristic has a lagging power factor, meaning the current waveform lags the voltage waveform. When switching off an inductive load, the inductance causes a back EMF which can result in arcing across the contacts.

NOTE: It is recommended not to use the relay to directly switch a load with a lagging power factor of lower than 0.6. In such cases, a suitable external contactor should be used.

Motor Loads

Motors are a special case of an inductive load. In addition to the inductance of the windings which cause arcing when switching off, the inertia of the rotor creates a higher current at startup.

Unlike the inrush for a capacitive load, the motor startup current is a lower magnitude (typically 5-6 times the run current) but much longer duration. In addition, switches for motor loads need to consider the possibility that the rotor may stall. When this happens the current can be 4-5 times the normal run current (this is referred to as the "locked rotor" current).

For these reasons the rated motor load for the relays is lower than the other switching ratings. For Australian /NewZealand installations, the M - rating of the relay channel refers to the maximum locked rotor current for an attached motor.

DC Loads

DC loads are not recommended, however the relay may be used with resistive DC loads up to 24Vdc.

Minimum Load

The contact material used in power relay contacts requires a minimum current flow when making or breaking the circuit to maintain a low-resistance contact. The minimum recommended load current is 100mA @ 12Vdc. Even though this current corresponds to a power of 24VA in a 240V system, in most cases the initial inrush current for lighting loads less than this is sufficient to maintain low contact resistance.

Dry-Contact Inputs

Whilst the relay contacts are voltage free, due to the minimum load requirement, they may not be suitable for low-current applications such as switching dry-contact inputs.