Contactor Procurement Guide: Defining Requirements & Evaluating Suppliers Buying a contactor sounds simple until the wrong one turns up: the wrong coil voltage, an auxiliary contact configuration that doesn't match the panel, or a supplier that can't confirm genuine stock. For procurement teams specifying contactors across multiple sites — from oil & gas facilities to general manufacturing — a structured approach to requirements and supplier evaluation avoids costly re-orders and installation delays. This guide walks through how to define your contactor requirements properly and how to evaluate a UK contactor supplier before you commit to an order. Need help specifying or sourcing a contactor for your site? Talk to Our Expert Team Defining Requirements Before you send an RFQ to a contactor uk supplier, lock down the electrical basics: coil voltage (AC or DC, and the exact voltage — 24V DC, 110V AC and 230V AC coils are the most commonly mis-ordered), pole count (3-pole for standard three-phase motor circuits, 4-pole where a switched neutral is required), and current rating (AC-3 rating for motor duty, not just the nameplate thermal current). Confirm the auxiliary contact configuration you need (1 NO, 1 NC, or a combination) since retrofitting auxiliary blocks after installation adds cost and downtime. For oil & gas facilities and other hazardous or high-vibration environments, also confirm the enclosure and mounting requirements and whether a UL 489-rated or IEC 60947-4-1-rated device is specified by your site standard. AC-3 and AC-4 Duty Ratings — Why the Nameplate Current Isn't the Whole Story One of the most common specification errors on a contactor RFQ is quoting only a thermal current rating and ignoring the duty category. A contactor's AC-3 rating describes its suitability for switching a running motor — making current at up to six times full-load current on start-up, and breaking current at normal running load. This is the rating that matters for standard motor starting and stopping duty, such as pumps, fans and conveyors, where the motor is switched off while still running. An AC-4 rating is a harder duty: it covers plugging (reversing while running) and inching/jogging applications, where the contactor makes and breaks at full starting current repeatedly, generating significantly more contact wear and heat. A contactor sized correctly for AC-3 duty on a simple start/stop pump application may be undersized and short-lived if redeployed on an AC-4 jogging application without re-checking the rating. When you send an RFQ, state the actual duty type expected in service, not just the motor's full-load amps — a supplier who only asks for the latter isn't giving you a properly specified part. Contactor vs Residual Current Device (RCD): Two Different Jobs Procurement teams new to switchgear sometimes assume a contactor and an RCD are interchangeable or that one supersedes the other — they're not, and a panel typically needs both for different reasons. A contactor is a control device: it switches the motor or load circuit on and off, either manually via a control signal or automatically from a PLC or control system, and it has no built-in fault-sensing function of its own. An RCD (residual current device) is a protection device: it continuously monitors for earth leakage current and disconnects the supply if a fault to earth is detected, protecting personnel from electric shock. In a typical motor circuit, the contactor handles routine switching duty while upstream protection — an MCB, MCCB or RCD depending on the circuit design — handles fault conditions. Specifying one in place of the other is a design error that either leaves a circuit without shock protection or without proper switching control, so treat contactor and RCD requirements as separate line items on any panel build RFQ. Where the Contactor Fits in a Motor Starter Circuit A contactor rarely operates alone. In a direct-on-line (DOL) starter, the standard arrangement is an isolator or switch disconnector for safe isolation, a motor protection circuit breaker or fuse for short-circuit protection, the contactor itself for switching duty, and a thermal overload relay in series to protect the motor from sustained overload current. Getting the contactor's AC-3 current rating matched to the thermal overload relay's adjustment range is worth checking at RFQ stage — a contactor sized correctly for the motor but paired with a mismatched overload relay range will either nuisance-trip or fail to protect the motor as intended. If you're specifying a full starter assembly rather than a like-for-like contactor replacement, it's worth reviewing overload relay selection alongside the contactor order rather than as a separate purchase. Supplier Evaluation Criteria Not every contactor uk supplier operates to the same standard, and price alone is a poor way to differentiate between them. The criteria below are what we'd suggest scoring a shortlist of suppliers ag…