1. Introduction
Relay ratings are a source of endless confusion – and a place where manufacturers can be less than honest.
For relays that switch mains voltages and currents: Let’s do a dive into relays: what they do, how they work, what makes them fail, and how ratings are (or should) be stated.
2. What this relay thing all about then?
A RELAY is an electro-mechanical device that operates as a switch.
When energised, a coil causes a magnetic field to be created, and this makes a movable armature change position. The armature operates the electrical switch to a “closed” position.
When the relay is not energised, a spring causes the armature to return to it’s resting position; and this in turn will cause the switch to adopt the “open” position*.
Let’s unpack this:
- Electro-mechanical: means it has electrical things, and mechanical things. We can assume then, that there are perhaps moving parts.
- Operates as a switch: means that it can pass an electrical current, and therefore allow the current to pass from a source to a load, or interrupt the current.
- Magnetic fields and other stuff is the means in physics by which the armature can be moved. We can ignore this detail.
- A movable armature is simply a mechanical piece that can move between two positions.
- Physical movement of the armature can be coupled to an electrical switch to open or close a circuit.
- There is a resting position (the “normal” position) which is when the relay coil is not energised.
The simple way: A relay is an electrically operated switch.
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* Pedants corner: there are other types of relay, including normally closed contact types; and also latching relays. These are variations on the major theme and we won’t go into them here.
3. Relay Properties
Because a relay acts as a switch, every relay has at least these two important properties:
- The voltage, current, etc needed for the coil to cause the relay to change state.
[What must be put in, in order to get the relay to change away from the normal or un-energised state?]
- When the relay switches a load: what kind of load can it switch, for how many switching cycles, and under what conditions?
We won’t concern ourselves any further with point 1 – this is a matter for design engineers, drive circuits and other such exotic matters.
Point 2 is the whole reason for this article:
- What types of loads are there?
- What do different load types do to relays, their switching behaviour, and to relay lifetime?
- And finally: how can we read a relay product rating label and gain an understanding of what that relay can withstand?
4. Load Types
There are many kinds of loads: lamps (and many kinds of these), motors, resistive heaters, and so on.
Each of these has different properties, but all can be separated into three fundamental types:
- Resistive load;
- Capacitive load; and
- Inductive load.
Now we need to get into a little bit of basic engineering. The technical term is “step response”, which can be simplified to: What happens when you turn the load on, and what happens when you turn it off?
Setting aside relays for a moment and assuming a perfect switch, we need to look at the voltage and current for each load type to see what happens at the instant in time of switch on and switch off.
Just to make things a bit more complicated, we need to also introduce the idea of source impedance. This can be simplified to a small resistor, representing the resistance in the mains wiring between a big generator capable of supplying essentially unlimited current at a typical mains voltage of 230 to 240 V*.
That source impedance is what helps to limit things like inrush current.
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* Another pedants corner: we’re initially ignoring the distinction between ac and dc, as well as peak and rms voltages. The point here is to explain the principle: therefore to be right in principle even if not 100% technically precise.
4.1 Resistive Load
Mains powered resistive loads are fairly infrequent. Examples are:
- Resistive hot water heater element; and
- Incandescent lamp (but just to make life interesting, these lamps have a resistance that changes as they get hot).
Resistive loads are the most benign loads for a switch or relay to control.
Switching a resistive load ON has the behaviours shown below, where Vload is the voltage that appears across the load resistance, and Iload is the current in the load resistance.
Or putting it in words: At switch on, the voltage appears across the load, and the load draws a constant current which flows from the instant of switch-on.
Switching a resistive load OFF is very simple: the voltage and current both change to zero when the switch is opened. After the switch off, there maybe arcing over the relay contact until the mains power reaches a current zero-crossing.
LESSON: We can conclude that for Resistive loads, the relay contacts are stressed by arcing that happens at switch-off.
4.2 Capacitive Load
Capacitive (shunted) loads are common in modern mains power switching. Examples are:
- LED lamp drivers;
- Fluorescent lamps (due to the power factor correction capacitor; or with electronic ballast); and
- Electronic transformers for downlights.
A Capacitor has these properties:
- It can store energy;
- Changes (ac signals, impulses, steps) try to pass through; and
- A static (dc) condition is blocked.
Switching a capacitive load ON has the behaviours shown below, where Vload is the voltage that appears across the capacitor, and Iload is the current in the load capacitor:
Again, to be pedantic, the load will have more to it than just a capacitor – for instance, other circuitry to do something useful – but we’re considering a load that is mainly capacitive.
Switching a capacitive load OFF has the behaviours shown below:
The maths to derive the shape of the voltage and current is complex and not important to show here.
The important point is that:
- At switch ON: For a Capacitive load, the voltage rises with an exponential shape and the current starts instantly at a very high value and decays. This is the Inrush current.
- At switch OFF: The voltage decays away gradually due to other aspects of the load. The current delivery into the load immediately becomes zero.
LESSON: We can conclude that for Capacitive loads, the relay contacts are stressed by inrush current which happens at switch-on.
4.3 Inductive Load
Inductive loads are typically coils, motors, and similar. Examples are:
- Iron core and toroidal transformers (but not electronic transformers, which are frequently capacitive);
- Motors; and
- Relays, including some contactors.
Motors have other factors to consider also; and these need separate special consideration.
An Inductor has these properties:
- It can store energy;
- Changes (ac signals, impulses and steps) are hard to pass through; and
- A static (dc) condition is passed.
Switching an inductive load ON has the behaviours shown below, where Vload is the voltage that appears across the load inductor, and Iload is the current in the load inductor:
Like the capacitive case, the load will have more to it than just an inductor – for instance, other circuitry to do something useful – but we’re considering a load that is mainly inductive.
Switching the inductive load OFF has a counter-intuitive effect. The stored energy wants to get out of the inductor, so an induced voltage appears across it (the “back EMF”) – the magnitude depends on the inductance, losses in the inductor and a few other things.
The effect is that the voltage across the switch (the relay contacts) is the source voltage V + the back EMF voltage. This sum is often substantially higher than the voltage V from the voltage source. For a mains voltage of (say) 240 V, it’s common to see 400 V or more across the switch for a short time.
The high voltage across the switch (relay contacts) at switch-OFF time, as the contacts are being opened, may causes an arc. The ionised air in the arc causes a low resistance path which sustains the arc, so current continues to flow (through the arc) to the load.
In dc systems, the back-EMF from a relay can be managed by use of a diode (normally reverse biased, but conducting when the switch opens). This creates a temporary short-circuit that allows the energy to be dissipated.
When the supply voltage is an ac source such as mains power, the mains voltage waveform and current waveform are sinusoidal. A diode can’t be used, and instead the arc will be quenched when the current flowing through the arc reaches a zero-crossing of the current waveform.
The arc over the relay contacts degrades the contacts, typically causing pitting. Over time, the contacts can have a higher on-state resistance or will eventually become welded together or overwise destroyed.
Like a capacitive load, the maths to derive the shape of the voltage and current is complex and not important to show here.
The important points are:
- At switch ON: For an Inductive load, the voltage is applied instantly, and the current rises slowly. There is no concern for inrush current with inductive loads.
- At switch OFF: The back-EMF causes a voltage to appear across the switch that can damage the switch, or for a relay, cause arcing until an ac zero crossing quenches the arc.
LESSON: We can conclude that for Inductive loads, the relay contacts are stressed by arcing that happens at switch-off.
The special case of Motor Loads
Motors are highly inductive, so as we saw above the switch-off operation can lead to relay contact damage.
Motors also have a separate case to consider. This is that when started, the current drawn is higher than the running current – that is, they also have an inrush current which can last an extended period of time until the motor is running at its normal speed. Most standards used for compliance testing use a “locked rotor” current of 2x to 6x to normal running current. Compared to capacitive load this is generally modest.
The higher starting current will cause contact heating; and the effect of this is generally relevant only to small relays where the heading may degrade the contacts.
LESSON: We can conclude that for Motor loads, the relay contacts are stressed mainly by the arcing that happens at switch-off.
5. Relays are not perfect switches
5.1 When contact damage happens
A relay contact is damaged by the arc that happens when the contacts open.
There’s a clear and simple case – when the relay is switched off – that is, the contact opens.
There’s a less clear case, which is that when the relay contacts close. During close, the contacts tend to bounce. During the bounce, there’s an arc as well. This leads to contact damage on closing (due to the bounce). This is why high inrush currents lead to relay degradation.
Conclusion: Relay contacts are damaged due to arcing – both during the turn-on bounce and at turn off.
5.2 Contact Materials
Relay manufacturers use different contact materials (such as silver, silver tin oxide, and other more exotic types) to try and manage the performance, resistance and arc tolerance of their products.
Manufacturers can adjust the thickness and other properties of the contact materials, and the ways they do this is generally a trade secret.
The only way to determine the performance is to read the product data sheet. Any parameter not stated is probably not stated for a reason!
6. Relay Ratings
The ratings of relay need to take account of the circuit switching ac or dc, and for ac loads: the load type, voltage and current.
Having separate ac and dc ratings is straightforward:
- When switching dc and opening the contacts, there is no way to stop an arc, so any rating needs to ensure an arc can’t be created in the first place;
- When switching ac and opening the contacts, the zero crossing of current means that – sooner or later – the arc will be quenched and therefore a higher current rating is possible. This rating will be based, in part, on the lifetime of the contacts.
For mains power ac switching applications we can ignore all aspects of dc ratings.
The following sections describe what a well-specified relay product should state in its ratings.
If the information shown below is not presented, the relay may not be suitable for the intended application or may fail after only a small number of switching operations.
6.1 Current for ac load switching
A relay should state the ac current rating and the load type that this applies to.
Complete specification of current rating should cover the current rating for:
- Resistive loads;
- Inductive (non-motor) loads such as transformers;
- Capacitive loads such as LED and fluorescent; and
- Motor loads.
An ac relay rating is typically given at a voltage and current – for example:
240V, 10A
100 – 240V, 10AX
240V 10A capacitive.
When “X” appears in the rating, this is a convention that applies to switching fluorescent lamps, which are highly capacitive. A relay with an “AX” rating is generally a sign that it will be robust for capacitive loads.
6.2 Motor Loads
As shown above, motors are highly inductive but also have high startup currents.
A relay will have a much lower current rating for motors, when compared to resistive loads, typically around 1/6 to 1/5 that of the resistive rating.
If a relay can be used for motor loads, the data sheet or rating should state this, along with the current limit.
6.3 Inrush
A well-specified relay will include a statement of the inrush current it can withstand. This applies to switch-on for capacitive loads.
To be properly specified, the inrush rating should state both a current and a time.
For comparison purposes: higher current numbers are better. The time is important, but less so than the inrush current stated.
A relay that does not state an inrush rating is likely to have a poor lifetime when switching capacitive loads.
6.4 Number of Switching Cycles
When relays fail, it is usually because of contact damage: the contacts need to carry the electrical current. Their highest stresses are:
- At switch on: for capacitive loads and motor loads
- At switch off: for inductive loads and motor loads
A well-specified relay will include a statement of the number of switching cycles it can achieve.
Some relays may quote a different number of switching cycles for different load types, for example:
- 100,000 cycles at 10A resistive
- 20,000 cycles motor load 2A
- 50,000 cycles 10A capacitive
A relay is not suitable for a load type if the number of switching cycles for that load type is not stated.