Power relay

01

Match the relay to the load, not just the amps

ARATAS (formerly Omron Components) explains that contact ratings vary with the voltage applied and the load type and are indicated for resistive loads. Motors and incandescent lamps draw inrush current; a lamp load draws approximately 10 times its steady-state current at switch-on. The UL/CSA TV rating tests inrush withstand with a tungsten lamp load over 25,000 operations, for example TV-5 at 78 A inrush and 5 A steady, and TV-8 at 117 A and 8 A. [1]

DC loads are harder on contacts: without the zero crossing of AC, the arc lasts longer and contacts wear faster; connecting contacts in series increases the total gap to help control the arc. At very small loads, contact resistance can build up through film formation, and twin contacts are more reliable than single contacts. [1]

Causes of contact welding listed by ARATAS include lamp inrush, short-circuit current, vibration, chattering when voltage drops and too high a switching frequency; abnormal contact wear with motor, solenoid and lamp loads calls for surge-absorbing measures. [1]

Give the supplier the load type (resistive, lamp, motor, capacitive, inductive), voltage AC or DC, steady and inrush current, switching rate and required life, coil voltage and drive, mounting, insulation and approvals required.

02

Read a power relay datasheet: a worked example

Hongfa's HF115F datasheet shows the values a buyer should compare. The 1-pole version is rated 12 A or 16 A at 250 VAC resistive, with 440 VAC / 300 VDC maximum switching voltage and 3,000 VA / 4,000 VA maximum switching power; contact resistance is 100 mΩ max at 1 A 6 VDC; mechanical endurance is 1×10⁷ operations, and electrical endurance for the 1H3B type is 1×10⁵ operations at 16 A 250 VAC resistive, room temperature, 1 s on and 9 s off. [3]

Insulation: 1,000 MΩ at 500 VDC; dielectric strength 5,000 VAC for 1 min between coil and contacts and 1,000 VAC across open contacts; 10 kV surge between coil and contacts; operate time 15 ms max and release time 8 ms max; ambient −40 °C to 85 °C. [3]

Editorial calculation: the 24 VDC coil is listed at 1,440 Ω, so it draws 24 ÷ 1,440 ≈ 16.7 mA and 24² ÷ 1,440 = 0.4 W, matching the approximately 400 mW coil power. Its pick-up voltage of 16.8 V max is 70% of nominal and its drop-out of 2.4 V min is 10%. [3] The 1×10⁵-operation electrical rating at one operation per 10 s represents about 278 hours of testing; at 100 operations a day it would last about 2.7 years, but only under the stated resistive load. Electrical durability is counted at the rated load, so test under your actual load. [1]

The same datasheet says products with rated coil voltage of 48 V or more need measures such as a parallel diode to prevent coil overvoltage.

03

Ask which IEC 61810-1 procedure the relay was type-tested to

IEC 61810-1:2015 "applies to electromechanical elementary relays (non-specified time all-or-nothing relays) for incorporation into low voltage equipment (circuits up to 1 000 V alternate current or 1 500 V direct current)". It defines basic functional and safety requirements, verified by type tests, for uses from industrial equipment and household appliances to medical equipment, telecommunications and railways. The fourth edition, published on 25 February 2015, replaced the 2008 third edition and introduced two test procedures: procedure A, as in edition 3, and procedure B, "reflecting the assessment according to North American requirements". [4]

Hongfa lists the HF115F at 12 A or 16 A at 250 VAC resistive with 1×10⁵ electrical operations for one type [3], and ARATAS notes that ratings are stated for resistive loads and that lamp loads draw about 10 times steady-state current at switch-on [1]. Ask the supplier which edition and procedure of IEC 61810-1 the relay was type-tested to, which contact load category the electrical endurance figure refers to, and the certificate number.

For products sold in North America, edition 4's procedure B and Clause D.1 application tests are the parts that reflect North American requirements [4]. ARATAS describes the UL/CSA TV rating as an inrush test with a tungsten lamp load over 25,000 operations [1]; if the load is a lamp or motor, ask for that rating as well as the IEC resistive rating.

Editorial calculation: the 1 000 V AC and 1 500 V DC scope limits are well above a 250 VAC relay rating; at 16 A and 250 V a resistive load is 4,000 VA, matching the HF115F maximum switching power figure. [4] [3]

Values are from the Standard Norge catalogue pages for IEC 61810-1:2015 and IEC 61810-1:2008; the full standard was not reviewed.

04

Compare what suppliers offer

Suppliers linked to this page list power relays alongside other relay types; each card links to the page reviewed.

Hongfa lists power relays from 5 A to 40 A among its other relay families; Forward, Meishuo, Qianji, HWRELAY and Shenler list power relays with general-purpose, PCB, industrial or automotive relays and sockets; Naidian lists power relays with time and solid-state relays; Mycoil lists general power relays with EMI filters and transformers; RHTECP lists power relays in an electronic components catalogue.

05

Trace the relay components

This is a functional reference built from the sources cited; it is not one maker's parts list.

06

Fill the specification fields with values for your load

A power relay quotation needs values for your load and circuit. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the load, coil and life data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Load: [type; voltage AC/DC; steady current; inrush current]
Switching: [operations per day; required life]
Coil: [voltage; drive; suppression]
Contacts: [form; material]
Mounting: [PCB; socket; sealing]
Insulation: [dielectric; creepage; approvals]
Environment: [temperature; humidity; vibration]

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