Relay contact materials – does it really matter?

If a relay ‘works’, why worry any further about its contact materials? Based on an article by Norman Carnt of Finder UK, this article explains why access to the right contact material may be important.

It is common for relay users to take a standard product and with it, a standard relay contact material. Often, they’re perfectly happy – never have a problem – and don’t give alternative contact materials a second thought. There are some applications, however, in which alternative relay contact materials can be very useful.

Switching loads of up to 50 A is generally possible with industrial relays, whilst higher currents are usually the province of contactors. The principal contact materials used for relays with nominal contact ratings within the range 5 to 50 A are most commonly, Silver Nickel, Silver Cadmium Oxide and Silver Tin Oxide.

Silver Nickel (AgNi) has been around for “almost ever”. The relatively small nickel content (10%) is primarily to mechanically harden the silver and increase the resistance to electrical erosion of the contact faces, therefore making it that much more robust under heavier electrical load. It is ideal for resistive loads at the full nominal current rating of the contact, and for other loads where the load current is not so high. It is an economical and good-performing general-purpose material and quite often the standard material for many power relays. 

Silver Cadmium Oxide (AgCdO) has been popular for perhaps 50 years, particularly for its very good performance when switching inductive and motor loads. Contact material erosion is lessened and in particular, the material has an improved resistance to contact welding under conditions of short-term high peak inrush currents that result from switching large contactor coils, incandescent lamps and small motors.

Its use has for some time been limited by the “RoHS” European Directive 2002/95/EC. In its first edition, Cadmium was prohibited completely, but a further revision allowed its use in electrical contacts. The so-called “RoHS II” 2011/65/EU still permits this but establishes a deadline (unless any further revision in the next months) for general use until the end of July 2025. For this reason, Finder will cease production of any products, including relays containing Silver Cadmium Oxide at the end of 2023.

Silver Tin Oxide (AgSnO2) is a more recent innovation, and like Silver Cadmium Oxide (AgCdO) is produced by a powder/sintering process – unlike Silver Nickel which is a true alloy. The incredibly fine grinding of Tin Oxide into sub-micron particles, the even dispersion of this within the powdered silver, and the final high pressure forming to make the contact is a procedure that requires the most meticulous process control. In the early days of Silver Tin Oxide, the quality control, and therefore the performance, of these sintered materials was not always as consistent as it needed to be. However, today the high performance of Silver Tin Oxide can be relied on, and nowhere more so than in the handling of large peak inrush currents primarily caused by power factor correction capacitors associated with fluorescent and other gas discharge lamps, and also the input circuitry associated with modern energy-saving lamps, CFL or LED.

The fundamental problem with switching capacitive loads is that there is virtually no intentional current limiting in the circuit. Instantaneous currents are therefore limited only by source and line impedance and will be in the order of several hundred, if not a few thousand amperes. Similar peak currents occur when powering up switch mode power supplies and variable speed inverter drives. Not surprisingly, contact welding has historically been a big problem in these applications, but with careful evaluation of the application against the known performance of the relay under such conditions, it is often possible to predict the likely improvement that a change to Silver Tin Oxide will bring. 

Nevertheless, there are choices that the user can, or indeed should, exercise when it comes to the subject of contact materials. Broadly speaking one should avoid using relays with power contact materials, as the characteristics that made them good power switchers, tend to work against reliable low-level switching. But occasionally there arises the need to switch both power and low-level circuits; then the only realistic option would be to select a power relay with the apparent anomaly of having gold-plated contacts. “Anomaly” since it makes little sense to gold plate power contacts, as gold is expensive and would simply be burnt off under power-switching conditions. “Apparent” because we know that there will be the odd occasion when this solves a mixed switching application with reliability at both ends of the scale. There is, however, a very important aspect to this. The gold must be plated to a significant thickness – avoiding any suggestion of using a gold flash that is typically of the order of 0.2 microns.

This is not just because such a thin coating will mechanically wear through within a few thousand operations; so do not be mistaken into thinking that because the relay only operates in your application once a month all will be well – it won’t! For reliable low-level switching a gold plate will be excellent, but a gold flash is likely to be worse than bare silver! The reason for this is a very interesting mix of physics and chemistry at play – but unfortunately, the in-depth explanation is beyond the scope of this article.

Typically a 7 A medium power relay with Silver Nickel contacts has a minimum switching specification of 5 V / 5 mA / 300 mW. However, this relay is also available with gold-plated contacts, and then the minimum switching specification revised values become 5 V / 2 mA / 50 mW.

If a much lower voltage must be reliably switched, consider two contacts in parallel. This dramatically lowers the minimum switching load – two parallel gold contacts make it possible to handle loads down to 0.1 V / 1 mA / 1 mW.

It may be useful to appreciate that statistically, the unreliability of two contacts in parallel is equal to the unreliability of the single contact raised to the power of two. So, just to illustrate the maths, a 1% unreliable switching circuit becomes 0.01% unreliable – i.e., a 100x improvement in reliability. And for three contacts in parallel, the unreliability would be raised to the power of three – a 10,000x improvement in reliability!

This is not just because such a thin coating will mechanically wear through within a few thousand operations; so do not be mistaken into thinking that because the relay only operates in your application once a month all will be well – it won’t! For reliable low-level switching a gold plate will be excellent, but a gold flash is likely to be worse than bare silver! The reason for this is a very interesting mix of physics and chemistry at play – but unfortunately, the in-depth explanation is beyond the scope of this article.

Typically a 7 A medium power relay with Silver Nickel contacts has a minimum switching specification of 5 V / 5 mA / 300 mW. However, this relay is also available with gold-plated contacts, and then the minimum switching specification revised values become 5 V / 2 mA / 50 mW.

If a much lower voltage must be reliably switched, consider two contacts in parallel. This dramatically lowers the minimum switching load – two parallel gold contacts make it possible to handle loads down to 0.1 V / 1 mA / 1 mW.

It may be useful to appreciate that statistically, the unreliability of two contacts in parallel is equal to the unreliability of the single contact raised to the power of two. Just to illustrate the maths, a 1% unreliable switching circuit becomes 0.01% unreliable – i.e., a 100x improvement in reliability. And for three contacts in parallel, the unreliability would be raised to the power of three – a 10,000x improvement in reliability!