Reliable switching with low-energy contacts

Hello, the standard reference needs clarification first. A separate UL publication numbered “UL 60947-5-4” could not be verified through the official catalog research. The verified publication relevant to this subject is IEC 60947-5-4, which addresses special tests for assessing the performance of low-energy contacts. The technical discussion below follows that IEC scope.

It should not be described as a general safety standard for PLCs or programmable relays. Among the standards relevant to PLC hardware, IEC 61131-2 addresses functional and EMC requirements, while IEC 61010-2-201 addresses safety requirements for the relevant control equipment. Using a mechanical contact at an input does not place the entire controller within the scope of 60947-5-4.

Why can a smaller current be harder to switch?

A relay or pushbutton can close mechanically without transmitting an electrical signal reliably on every operation. Contamination and insulating films on the contact surface can cause problems, particularly at low voltages and currents. When the load provides little electrical cleaning action, intermittently high contact resistance or signal interruption can occur.

The ability to switch a substantial current and the ability to transmit a very small signal reliably are different selection criteria. A contact marked 5 A or 10 A should not be assumed suitable for an electronic input drawing a few milliamperes on that information alone. Consider the minimum applicable load as well as maximum capacity.

The actual scope of the verified standard

The standard focuses on separable contacts operating at low energy in applications such as control circuits. It assesses electrical conduction under defined conditions. Results do not constitute general approval of PLC software, processor speed or an entire machine.

The verified consolidated publication is IEC 60947-5-4:2002+AMD1:2019 CSV. Its scope identifies regions at or above 10 V, typically 24 V, and below 10 V, typically 5 V. This voltage distinction does not establish that every 24 V circuit or contact is suitable for low-energy operation.

Separating contacts, control devices and motor circuits

Broader requirements for electromechanical control devices are addressed by IEC 60947-5-1. The published US standard UL 60947-5-1 also concerns electromechanical control devices, with UL 60947-1 providing general rules. Check the scope and edition of each IEC or UL publication separately; similar numbers do not establish equivalence.

A contactor's main power contacts and its auxiliary contact reporting status to a PLC do not switch the same load. UL 60947-4-1 is relevant to motor control and electromechanical contactors. An auxiliary contact used in a small-signal circuit still needs suitable load ratings and supporting manufacturer information.

When selecting a limit switch, examine its contact material and minimum load information alongside mechanical travel, operating frequency and environmental conditions. NO and NC identify the normal state; they do not establish minimum switching capability. Proximity switches covered by UL 60947-5-2 may instead use semiconductor outputs, which require a different assessment from mechanical contacts.

A practical PLC input example

Suppose a hypothetical input circuit draws 2 mA at 24 V DC. A simplified power calculation gives 24 × 0.002 = 0.048 W, or 48 mW. This illustrates the small signal involved; obtain the actual input thresholds, current characteristic and filtering behavior from its data sheet.

Selecting a relay for this circuit requires more than reading “5 A, 250 V AC.” Review the manufacturer's minimum switching load, contact construction and any reliability reference data. The power supply's nominal voltage alone does not establish the contact's operating point either; assess the current through the contact and the connected load.

Gold contacts and minimum-load specifications

Gold-plated or gold-alloy contacts, twin contacts and bifurcated crossbar arrangements can be used for small signals to support dependable electrical contact. However, gold plating alone is not an unconditional reliability guarantee; the contact construction and application limits still need to be considered together.

Previously switching a higher-energy load can alter the surface and coating of a contact intended for small signals. A used power relay should therefore not automatically be reassigned to a small-signal task, nor should different load types be alternated on the same contact without supporting manufacturer information.

If a minimum load or failure-rate specification is identified as a reference value, read the associated test and environmental conditions. One small-load figure does not guarantee identical results in every environment or over every number of operations. This is why verification with the actual load and operating conditions matters.

Interpreting tests and measurements

An assessment should identify the applied contact voltage and current, load characteristics, switching arrangement, environmental conditions and failure criteria. This is a general assessment framework; a claim of standard compliance requires the detailed method and report applicable to the publication. A multimeter continuity indication does not establish reliable small-signal operation on every cycle.

For low-resistance measurements, a four-wire Kelvin connection can reduce the influence of test-lead resistance. Measurement voltage and current matter as well as the connection method. Since measurement conditions can affect the contact surface, a higher-current resistance measurement should not replace a low-energy operating test. Measure resistance on a de-energized circuit with suitable isolation.

An intermittent field signal is not always caused by the switching contact. Check cable connections, terminal security and mechanical vibration too. UL 60947-7-1 is a separate part concerning terminal blocks; assessing a fixed terminal connection is different from assessing a low-energy contact that repeatedly opens and closes.

Contact reliability and a safety function are different assessments

Successful small-load switching does not, by itself, establish SIL, PL or safe-stop capability. Proximity devices with specified fault behavior are addressed by IEC 60947-5-3, while electrical emergency-stop devices with mechanical latching are addressed by IEC 60947-5-5. Evaluate a safety function in the context of the complete circuit and system.

For procurement or technical documentation involving UL certification, check the complete product designation, applicable standard, edition and conditions of use. A statement claiming compliance with “UL 60947-5-4” should not form the basis of technical acceptance without a verifiable publication and documentation for the product. Select a low-energy contact application using the correct scope and actual load data.

Related topics: UL 60947 family; control devices