{"id":4483,"date":"2026-09-12T09:00:00","date_gmt":"2026-09-12T09:00:00","guid":{"rendered":"https:\/\/www.paradigmcooling.co.za\/articles\/?p=4483"},"modified":"2026-09-12T09:00:00","modified_gmt":"2026-09-12T09:00:00","slug":"how-to-test-a-contactor-properly","status":"publish","type":"post","link":"https:\/\/www.paradigmcooling.co.za\/articles\/2026\/09\/12\/how-to-test-a-contactor-properly\/","title":{"rendered":"How to Test a Contactor Properly (Not the Way Most People Do It)"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">The standard test most technicians run on a contactor \u2014 ohms across the closed contacts, looking for something near zero \u2014 passes contacts that are actively destroying themselves. By the end of this article you&#8217;ll know why that test is unreliable, what to measure instead, and how to test the coil and contact circuits separately so you replace a contactor for a real, numbered reason.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Two Circuits, Two Failure Modes<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A contactor contains two electrically separate circuits, and most diagnostic errors come from confusing them. The <strong>coil circuit<\/strong> is the control side \u2014 a low-power electromagnet drawing milliamps, switched by the thermostat, pressure switches and overloads. The <strong>contact circuit<\/strong> is the power side \u2014 heavy silver-alloy contacts driven mechanically by the armature, carrying full load current to the compressor or fan.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The coil can fail with the contacts perfect; the contacts can be destroyed with the coil perfect.<\/strong> Test both, separately, every time \u2014 a contactor that pulls in reliably can still be running a scorched pole underneath.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Testing the Coil<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>First, resistance, with the power off.<\/strong> Isolate, lock off, prove dead per the OHS Act 85 of 1993 sequence, and disconnect one coil lead so you aren&#8217;t reading a parallel path. Measure across A1 and A2.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><thead><tr><th>Coil rating<\/th><th>Typical resistance<\/th><\/tr><\/thead><tbody><tr><td>24 V AC<\/td><td>5\u201360 \u03a9<\/td><\/tr><tr><td>110 V AC<\/td><td>100\u2013400 \u03a9<\/td><\/tr><tr><td>230 V AC<\/td><td>200\u20131500 \u03a9<\/td><\/tr><tr><td>24 V DC<\/td><td>100\u2013600 \u03a9<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">These ranges are wide because resistance depends on the contactor&#8217;s size as much as its rated voltage \u2014 the manufacturer&#8217;s figure matters more than the table. The failure signature is unambiguous: <strong>an open coil reads OL and is scrap. A shorted coil reads near 0 \u03a9 \u2014 under 2 \u03a9 where it should read hundreds \u2014 and is scrap. A coil at roughly half its expected resistance has shorted turns: it pulls in weakly, runs hot, and needs replacing<\/strong> even though it still closes.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Second, voltage at the coil terminals when the circuit calls.<\/strong> Leave the coil connected and measure A1 to A2 while the circuit tries to close it. <strong>Full control voltage with the contactor not pulling in means a jammed mechanism or an open coil. No voltage means the fault sits upstream<\/strong>, and the contactor is not yet implicated.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Pull-In and Drop-Out: Two Thresholds, Not One<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A contactor is not simply on or off. Under <strong>IEC 60947-4-1<\/strong> it must close reliably between <strong>85% and 110%<\/strong> of rated coil voltage, and drop out between <strong>20% and 75%<\/strong>. A 230 V coil must pull in by 195 V, a 24 V coil by 20.4 V; drop-out on a 230 V coil falls between 46 V and 173 V.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The gap between those numbers is where intermittent faults live. <strong>A coil sitting at 180 V \u2014 78% of rated \u2014 will often hold in once closed but will not reliably close from rest.<\/strong> The plant works after a manual push, then fails on the next automatic start, reported as &#8220;intermittent&#8221; because nothing looks wrong when you arrive. <strong>Measure coil voltage at the instant of pull-in, not at rest<\/strong> \u2014 the coil&#8217;s own inrush drags the supply down at exactly the moment it needs to close.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Chattering: Two Causes, One Measurement Apart<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A chattering contactor buzzes rapidly open and closed, from two distinct causes separated by one measurement.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Low control voltage.<\/strong> A coil draws six to ten times its holding VA during pull-in. If the supply can&#8217;t deliver that surge, voltage collapses, the coil releases, voltage recovers, and the cycle repeats several times a second. <strong>Measure coil voltage during pull-in: below 85% of rated (under 195 V on 230 V, under 20.4 V on 24 V) confirms this cause.<\/strong> Look upstream for an undersized control transformer, a long control cable, or a high-resistance joint.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>A cracked shading ring.<\/strong> An AC electromagnet&#8217;s flux crosses zero twice per cycle, which at 50 Hz would release the armature 100 times a second without correction. A copper ring in the pole face carries an induced current producing a phase-shifted flux so total flux never reaches zero. <strong>A cracked ring produces a loud 100 Hz buzz with correct coil voltage present.<\/strong> If voltage stays above 85% during pull-in and it still chatters, the magnet assembly is the fault, and the contactor is scrap regardless of coil resistance.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Contact Testing: The Measurement Most People Skip<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This is the test that decides whether a contactor stays in service, and it is not resistance. <strong>Measure voltage drop across each closed pole with the load running at normal current<\/strong> \u2014 one probe on the line-side terminal, the other on the load-side terminal of the same pole, contactor closed, motor running.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Below 0.1 V per pole is healthy. 0.1\u20130.3 V means pitting has started \u2014 record and plan replacement. 0.3\u20130.5 V is deteriorating fast. Above 0.5 V, replace now.<\/strong> These thresholds look small because dissipation is drop multiplied by current: a healthy 0.05 V drop at 24 A dissipates 1.2 W, but 0.9 V on the same pole dissipates 21.6 W inside a plastic housing never built to shed it.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Safety note:<\/strong> this is a live measurement at energised load terminals. Use a CAT III-rated meter, keep one hand clear of earthed metalwork, and confirm the enclosure is otherwise safe before you probe inside it.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Why Resistance Across Closed Contacts Fails You<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">A multimeter&#8217;s ohms range pushes about 1 mA through the circuit and resolves to around 0.1 \u03a9. <strong>A healthy contact reads 1 to 5 m\u03a9 \u2014 fifty times below that resolution<\/strong> \u2014 and the test leads alone contribute roughly 0.2 \u03a9. A pitted pole reading 0.2 \u03a9 looks like lead resistance and passes, yet at 25 A it drops 5 V and dissipates 125 W. <strong>Voltage drop under load tests the contact at the current it actually carries; cold resistance never gets there.<\/strong><\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Contact Welding<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Welded contacts leave the load energised with the coil de-energised \u2014 the plant runs and will not stop. <strong>Confirm it by de-energising the coil and measuring load-side voltage: full voltage present with 0 V across A1\u2013A2 means welded contacts. Isolate at the upstream isolator immediately.<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Welding is always a symptom: check repeated locked-rotor current from short cycling, a contactor sized on its AC-1 resistive rating rather than AC-3 motor duty, low coil voltage causing slow closure and bounce, and chattering. Fit the replacement, then fix whatever welded the last one.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Table: Contactor Fault Matrix<\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table><thead><tr><th>Symptom<\/th><th>Likely cause<\/th><th>What to measure<\/th><th>What confirms it<\/th><\/tr><\/thead><tbody><tr><td>Will not pull in, no sound<\/td><td>Open coil or no control voltage<\/td><td>Coil voltage A1\u2013A2 calling; cold resistance<\/td><td>Voltage present and coil reads OL, or no voltage at A1<\/td><\/tr><tr><td>Pulls in only when pushed by hand<\/td><td>Coil voltage below pull-in threshold<\/td><td>Coil voltage at the instant of pull-in<\/td><td>Below 195 V on 230 V, or 20.4 V on 24 V<\/td><\/tr><tr><td>Loud buzz, rapid opening and closing<\/td><td>Low control voltage, or cracked shading ring<\/td><td>Coil voltage during pull-in<\/td><td>Below 85% points to supply; above 85% points to the shading ring<\/td><\/tr><tr><td>Overload trips after 20\u201340 min, current reads normal<\/td><td>High-resistance pole heating the overload<\/td><td>Voltage drop across each pole under load<\/td><td>One pole above 0.5 V, others below 0.1 V<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Worked Example: A Cold Room Tripping Its Overload Every 20 to 40 Minutes<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A 400 V three-phase, 13 kW scroll compressor ran 20 to 40 minutes, tripped its overload, and restarted fine on reset \u2014 a pattern pointing at heat building up, not a starting fault. Phase-to-phase voltage read 398 V, 400 V and 399 V, and running current read 24.1 A, 23.8 A and 24.0 A against a 24 A nameplate \u2014 balanced and correct. The compressor was not overloaded, so something else was tripping it.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The 230 V coil measured 624 \u03a9 isolated, within band, and 224 V at pull-in \u2014 above the 195 V threshold, clearing the coil. Voltage drop across the three poles running told the real story: L1 0.04 V, L2 <strong>0.90 V<\/strong>, L3 0.05 V.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">L2 sat at nine times the 0.5 V replacement threshold \u2014 a contact resistance of 0.9 \u00f7 24 = 37.5 m\u03a9 against a healthy 1\u20135 m\u03a9, dissipating 21.6 W where the other poles dissipated 1.2 W. The overload, mounted directly beneath the contactor, was tripping on that pole&#8217;s heat plume, not motor current \u2014 a thermocouple on L2 read 103 \u00b0C after 25 minutes against 44 \u00b0C on L1. The site log showed the unit restarting immediately after every load-shedding recovery with no anti-short-cycle protection, at up to fourteen starts an hour against a design figure of six \u2014 eroding that pole with roughly 140 A of locked-rotor current per restart, a stress covered in <a href=\"https:\/\/www.paradigmcooling.co.za\/articles\/what-load-shedding-does-to-refrigeration-equipment\/\">what load shedding does to refrigeration equipment<\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Fitting a replacement rated for AC-3 motor duty and a 5-minute anti-short-cycle timer resolved it: rechecked, the three poles read 0.03 V, 0.05 V and 0.04 V, and the L2 terminal settled at 41 \u00b0C after 30 minutes. Condemned on &#8220;it trips the overload,&#8221; the compressor would have been replaced for nothing.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Final Thoughts<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A contactor earns its place in service on two tests: coil resistance and pull-in voltage on the control side, voltage drop under load on the contact side. Cold resistance across a closed pole substitutes for neither \u2014 it cannot resolve a healthy contact from one about to fail. Before probing any live terminal, confirm your isolation and proving procedure as laid out in <a href=\"https:\/\/www.paradigmcooling.co.za\/articles\/ghost-voltage-why-your-multimeter-reads-a-dead-wire\/\">ghost voltage and why your multimeter reads a dead wire<\/a>, and see the full sequence in our <a href=\"https:\/\/www.paradigmcooling.co.za\/hvacr-learning-library\/hvacr-electrical-diagnostics-the-complete-guide-to-electrical-troubleshooting-testing-and-fault-finding\/\">full electrical troubleshooting reference<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>The standard test most technicians run on a contactor \u2014 ohms across the closed contacts, looking for something near zero \u2014 passes contacts that are [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[36,18],"tags":[],"class_list":["post-4483","post","type-post","status-publish","format-standard","hentry","category-electrical-diagnostics","category-technical"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>How to Test a Contactor Properly (Not the Way Most People Do It) - &quot;Paradigm Cooling: Your Go-To HVAC &amp; 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