Use Explosion-Proof Relays for HVAC Systems with Flammable Refrigerants

Concerns about ozone depletion and greenhouse gas emissions have led to the increased use of flammable or mildly flammable refrigerants. This shift challenges designers of heating, ventilation, and air conditioning (HVAC) systems, including air conditioners, refrigeration equipment, and heat pumps, to eliminate ignition sources; one of the most obvious being electrical arcing in relays.

This article briefly outlines the reasons for the shift to flammable or mildly flammable refrigerants and the challenges it poses. It then introduces “explosion-proof” relays from Omron Electronics and shows how they support safety when using flammable refrigerants.

International environmental concerns and trends

Environmental regulations targeting ozone depletion and greenhouse gas emissions are prompting cooling system manufacturers to reconsider the use of refrigerants with high Global Warming Potential (GWP) and Ozone Depletion Potential (ODP), including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) (Figure 1).

Figure 1: There is a worldwide trend to restrict the use of refrigerants considered to have high GWP and ODP and to substitute them with alternatives. (Image source: Omron Electronics)

GWP is a dimensionless parameter that quantifies how much heat a gas traps in the atmosphere over a specified period relative to carbon dioxide. The GWP of carbon dioxide is 1. The GWP of other gases depends on their characteristics, including their heat-absorbing capacity, atmospheric lifetime, and the time frame considered.

The ODP is a measure of the destructive effects on the ozone layer relative to a reference substance. Trichlorofluoromethane (known commercially as R-11 or CFC-11) has a fixed ODP of 1.0. Lower ODP values indicate less ozone depletion; higher values indicate more ozone damage.

Past cooling system designs focused on the refrigerant’s cooling capabilities, but the current trend is toward using refrigerants with lower atmospheric impact, even though these refrigerants are flammable (Figure 2).

Figure 2: Shown are common refrigerants with their GWP and flammability ratings. (Image source: Omron Electronics)

The refrigerants shown within the dashed-line border are examples of refrigerants with good environmental characteristics but higher flammability than traditional refrigerants.

An A1 flammability rating indicates that the refrigerant is non-flammable. A2L-rated refrigerants are mildly flammable. By contrast, an A3 rating indicates that the refrigerant is highly flammable. An example of a highly flammable refrigerant in this chart is R290 (propane).

There is a general tradeoff between lower GWP and higher flammability ratings: a lower GWP generally corresponds to a higher flammability rating (Figure 3). Refrigerants with high GWP include CFCs and HCFCs.

Figure 3: Refrigerants with low GWP tend to be more flammable. (Image source: Omron Electronics)

Flammable refrigerants require special design consideration for electronics

The use of flammable refrigerants imposes certain limitations on electronic circuits used in cooling systems. The system must reduce the risk of fire or explosion by eliminating ignition sources. A common ignition source is electrical arcing in relays (Figure 4).

Figure 4: Ignition of flammable gases requires three elements: an ignition source (such as a relay switching arc), fuel (such as a combustible gas), and air (oxygen). (Image source: Omron Electronics)

A relay arc occurs when a relay's contacts open or close, and the intensity of the resulting spark varies with the current being switched. Arcing is more common in relays switching reactive loads, such as motors or capacitor banks.

The design of explosion-proof relays, with enclosures that prevent relay contact arcing from igniting surrounding flammable gases, makes them essential components in cooling systems that use flammable refrigerants. The use of explosion-proof relays reduces the risk of electrical arcing, providing a reliable and secure solution for operating electronics near low-GWP refrigerant gases.

International standards such as IEC/EN60079-1 and IEC/EN60079-15 specify the testing of relays intended for explosive or flammable environments.

Explosion-proof relays are certified in either of two ways. Relays with maximum current ratings of 16 amperes (A) or less, sealed for flux protection, are tested in a flammable gas environment. The contacts must be opened and closed 10 times without resulting in an explosion. This test ensures that the relay’s enclosure is designed to withstand the pressure generated by an internal explosion, prevent its transmission to the outside, and prevent ignition of the surrounding environment. This requires enclosures constructed from materials that provide high mechanical strength and long-term stability under severe operating conditions. This test does not eliminate the risk of ignition within the equipment; rather, it controls and contains it.

For relays switching more than 16 A, the enclosure must be fully sealed for explosion-proof certification. The relay’s mechanical design must comply with the IEC specification and be verified by an airtightness test. This relay characteristic prevents the flammable gas from reaching the ignition source.

Unsealed relays seal against solder flux and dust but are not airtight. They cannot be washed or coated. These relays offer longer life because they allow hot air to escape, keeping them cooler than sealed relays. In general, relay life decreases as switched load current and voltage increase.

Sealed relays, in addition to keeping gases out, also prevent the entry of cleaning solvents and can be washed or coated.

Omron power relays for use with flammable refrigerants

Recognizing the shift toward flammable refrigerants, Omron has introduced a broad range of explosion-proof single-pole single-throw (SPST), normally open (NO) relays compliant with IEC/EN 60079-1 and 60079-15 (Figure 5). These through-hole mount relays are rated for currents from 3 to 36 A at switching voltages up to 480 volts AC (VAC).

Figure 5: Shown are explosion-proof relays rated to handle a range of currents from 3 to 36 A at switching voltages up to 480 VAC. (Image source: Omron Electronics)

For example, the G5NB-1A4 DC12 is a fully sealed, general-purpose relay that conforms to IEC/EN 60079-15. It is rated to switch a maximum current of 3 A, with a maximum switching voltage of 250 volts VAC or 30 volts DC (VDC). The actuating coil voltage is 12 volts at 16.7 milliamperes (mA). The relay body measures 15.3 × 20.4 × 7.2 millimeters (mm) and is rated to operate over a temperature range of -40 to 85°C. The relay lifetime is rated at 5,000,000 mechanical operations and 200,000 operations at full load of 3 A at 125 VAC or 30 VDC.

The G2RL-1A DC24 low-profile relay is also intended for general-purpose applications, with a maximum current rating of 12 A and a maximum switching voltage of 440 VAC or 300 VDC. Its coil actuation voltage is 24 volts at 16.7 mA. The relay is sealed for flux protection only but qualifies for IEC/EN60079 due to its maximum current rating under 16 A, with appropriate testing. The relay body measures 15.7 × 29 mm × 12.7 mm, and its operating temperature range is -40 to 85°C. This relay has a mechanical lifetime of 20,000,000 operations. The lifetime under load is 50,000 operations at 250 VAC and 12 A, and 30,000 operations at 24 VDC and 12 A.

The G5Q-1A4 DC12 is a miniature 12 A power relay measuring 15.8 × 20.3 × 10.3 mm. It is a sealed relay that conforms to IEC/EN 60079-15 and can switch 277 VAC or 30 VDC. The coil voltage is 12 VDC at 16.7 mA, and it is rated to operate over a temperature range of -40 to 105°C. The rated mechanical lifetime is 10,000,000 operations. Under a 10 A, 125 VAC resistive load, the rated lifetime is 200,000 operations. For a 5 A, 30 VDC load, the rated lifetime is 200,000 operations.

For higher currents, the G5PZ-1A4-E DC18 is rated for 20 A and has a maximum switching voltage of 250 VAC. This is a sealed relay compatible with IEC/EM 60079-15. It is actuated by an 18 VDC coil at 29.4 mA, measures 25 × 24 × 10.5 mm, and has an operating temperature range of -40 to 70°C. The mechanical lifetime is 2,000,000 operations under a 250 VAC, 20 A load; the rated lifetime is 20,000 operations.

The G6QE-1A4 DC12 has the highest current rating of 36 A and a maximum switching voltage of 480 VAC. The coil is rated for 12 VDC at 117 mA. It is compatible with IEC/EN 60079-15 requirements. It measures 20.5 × 30.5 × 16 mm and is rated to operate over a temperature range of -40 to 85°C with a holding voltage of 35 to 80% of maximum. Its mechanical lifetime is 1,000,000 operations. For a 250 VAC, 20 A resistive load, the lifetime is 1,000 operations.

Conclusion

Explosion-proof relays have become a key component for ensuring the safe operation of cooling systems that use flammable refrigerants. Omron offers several families of sealed or unsealed explosion-proof relays with a range of current-handling capabilities, specifically engineered for cooling systems that use flammable refrigerants to minimize the risk of explosion or fire.