{"id":8240,"date":"2026-03-17T09:00:00","date_gmt":"2026-03-17T09:00:00","guid":{"rendered":"https:\/\/uplastech.com\/cable-concentricity\/"},"modified":"2026-11-01T15:04:54","modified_gmt":"2026-11-01T15:04:54","slug":"cable-concentricity","status":"publish","type":"post","link":"https:\/\/uplastech.com\/pl\/cable-concentricity\/","title":{"rendered":"Cable Concentricity: The Measurement That Predicts Failure"},"content":{"rendered":"<style>\n.upl-blog{font-size:17px;line-height:1.78;color:#1f2937}\n.upl-blog p{margin:0 0 18px}\n.upl-blog h2{font-size:26px;line-height:1.3;color:#0b2b33;margin:46px 0 14px;font-weight:700;letter-spacing:-.015em}\n.upl-blog h3{font-size:19.5px;color:#0d3b47;margin:28px 0 10px;font-weight:700}\n.upl-blog ul,.upl-blog ol{margin:0 0 20px;padding-left:22px}\n.upl-blog li{margin:0 0 9px}\n.upl-blog figure{margin:0 0 34px}\n.upl-blog figure img{width:100%;height:auto;display:block;border-radius:10px}\n.upl-blog figcaption{font-size:13px;color:#6b7280;margin-top:10px;line-height:1.5}\n.upl-blog .toc{background:none;border:0;border-radius:0;padding:0;margin:0 0 34px}\n.upl-blog .toc .lwptoc{margin:0}\n.upl-blog .toc h4{margin:0 0 10px;font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#0d5563}\n.upl-blog .toc ol{margin:0;padding-left:18px;font-size:14.5px}\n.upl-blog .toc li{margin-bottom:6px}\n.upl-blog table{width:100%;border-collapse:collapse;font-size:14.5px;margin:0 0 10px}\n.upl-blog th{background:#0d3b47;color:#fff;text-align:left;padding:11px 13px;font-weight:600;font-size:13.5px;border:0}\n.upl-blog td{padding:11px 13px;border:0;border-bottom:1px solid #e8ebed;vertical-align:top}\n.upl-blog tbody tr:nth-child(even){background:#fafbfc}\n.upl-blog .tablecap{font-size:12.5px;color:#6b7280;margin:0 0 30px}\n.upl-blog .myth{background:#fff8ef;border:1px solid #f6ddbd;border-radius:8px;padding:16px 20px;margin:0 0 26px;font-size:15.5px}\n.upl-blog .myth b{color:#a85c10}\n.upl-blog .callout{background:#f2f8f9;border-radius:8px;padding:18px 22px;margin:0 0 26px;font-size:15.5px;border-left:3px solid #14707f}\n.upl-blog .faq h3{border-top:1px solid #eceff1;padding-top:22px;margin-top:0;font-size:18px}\n.upl-blog .faq h3:first-of-type{border-top:0;padding-top:0}\n.upl-blog .cta{background:linear-gradient(135deg,#0b2b33,#0d5563);color:#fff;border-radius:12px;padding:34px;margin:48px 0 30px}\n.upl-blog .cta h2,.upl-blog .cta .cta-h{color:#fff;margin:0 0 10px;font-size:24px;font-weight:700;line-height:1.3}\n.upl-blog .cta p{color:#c9dee3;margin:0 0 20px;font-size:16px}\n.upl-blog .btn{display:inline-block;background:#e07a1f;color:#fff;text-decoration:none;font-weight:700;padding:13px 26px;border-radius:8px;font-size:15.5px}\n.upl-blog .rel{border-top:1px solid #e8ebed;margin-top:40px;padding-top:26px}\n.upl-blog .rel h4{font-size:13px;letter-spacing:.1em;text-transform:uppercase;color:#6b7280;margin:0 0 14px}\n.upl-blog .rel ul{list-style:none;padding:0;margin:0}\n.upl-blog .rel li{padding:11px 0;border-bottom:1px dashed #e8ebed}\n.upl-blog .rel a{text-decoration:none;font-weight:600;font-size:15.5px}\n@media(max-width:640px){.upl-blog h2{font-size:22px}.upl-blog table{font-size:13px}.upl-blog .cta{padding:24px}}\n\/* \u5355\u7bc7\u9875\u9690\u85cf WhatsApp \u95ee\u5019\u5361\uff0c\u53ea\u4fdd\u7559\u5706\u5f62\u6c14\u6ce1\uff0c\u907f\u514d\u906e\u6321\u4fa7\u680f\u8be2\u76d8\u63d0\u4ea4\u6309\u94ae\u3002\n   \u6ce8\u610f\uff1aClick to Chat \u7684\u70b9\u51fb\u903b\u8f91\u662f\u300c\u95ee\u5019\u5361\u5143\u7d20\u5b58\u5728\u5c31\u4e0d\u8df3\u8f6c\u300d\uff0c\u53ea\u9760 CSS \u9690\u85cf\u4f1a\u8ba9\u6c14\u6ce1\u53d8\u6210\u6b7b\u94fe\u63a5\u3002\n   Code Snippets #9\u300cUPL single post: WhatsApp bubble jumps straight to chat\u300d\u8d1f\u8d23\u628a\u8be5\u8282\u70b9\u4ece DOM \u79fb\u9664\uff0c\n   \u4e0b\u9762\u8fd9\u6761 CSS \u53ea\u662f\u79fb\u9664\u524d\u7684\u89c6\u89c9\u515c\u5e95\u3002\u5220 #9 \u524d\u5fc5\u987b\u5148\u5220\u8fd9\u6761\u89c4\u5219\u3002 *\/\nbody.single-post .ht_ctc_chat_greetings_box{display:none !important}\n<\/style>\n<div class=\"upl-blog\">\n<figure>\n<img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/cable-concentricity.jpg\" alt=\"Cable cross-section on the stage of an optical profile projector, the bright screen showing the magnified insulation ring visibly thicker on one side\" width=\"1536\" height=\"909\"><figcaption>A cable section on a profile projector. The ring is not centred, and the thin side is the one that decides the voltage rating.<\/figcaption><\/figure>\n<div class=\"toc\"><\/div>\n<h2 id=\"wall\">Concentricity is a wall problem<\/h2>\n<p>A cable is specified by its overall diameter and by its insulation wall, and of the two the wall is the one that matters. The diameter is what fits in a gland; the wall is what stands between the conductor and everything outside it.<\/p>\n<p>The wall is not uniform, and the variation has two parts. There is a random variation along the length, which is a process capability question, and there is a systematic offset, which is the conductor not being centred in the insulation. The offset is the one that dominates, because it is a bias: it makes one side thin for the whole length of the drum.<\/p>\n<p>The consequence is that the specified wall is a nominal and the real wall is a minimum somewhere. A 1.5 mm nominal wall with 20 percent eccentricity has a 1.2 mm thin side, and the cable is a 1.2 mm cable for every purpose that involves voltage.<\/p>\n<h2 id=\"breakdown\">Breakdown follows the thin side<\/h2>\n<p>Breakdown voltage against minimum wall is close to linear in the range that matters: 8.2 kV at 0.5 mm, 17.1 kV at 1.0 mm, 25.8 kV at 1.5 mm, 34.2 kV at 2.0 mm and 42.5 kV at 2.5 mm.<\/p>\n<p>Linear is the important part. It means a 20 percent loss of wall is a 20 percent loss of breakdown voltage, with no threshold and no safety net. A cable that was designed with a 25 percent margin and then made with 20 percent eccentricity has a 5 percent margin, and 5 percent is inside the tolerance of the test.<\/p>\n<p>The same arithmetic applies to the partial discharge inception voltage, which is the number that actually matters for a medium voltage cable. A void or a thin spot starts discharging at a voltage well below breakdown, and the discharge eats the insulation until the breakdown follows. That is why a cable can pass a breakdown test and fail in service two years later.<\/p>\n<figure>\n<img decoding=\"async\" src=\"https:\/\/uplastech.com\/wp-content\/uploads\/2026\/10\/cable-concentricity-chart.jpg\" alt=\"Breakdown voltage rising linearly from 8.2 kilovolts at 0.5 millimetres minimum wall to 42.5 kilovolts at 2.5 millimetres, and eccentricity rising from 2.8 to 17.1 percent as take-up tension increases\" width=\"1536\" height=\"909\"><figcaption>Left: breakdown voltage against minimum wall, 8.2 kV at 0.5 mm and 42.5 kV at 2.5. Right: eccentricity against take-up tension, 2.8 percent at 20 N and 17.1 at 100 N.<\/figcaption><\/figure>\n<h2 id=\"causes\">What actually moves the conductor off centre<\/h2>\n<p>The die centres the conductor, and everybody adjusts the die. That is often not the problem. The single largest cause of a systematic eccentricity on a real line is take-up tension, because a conductor pulled to one side leaves the die to one side and stays there.<\/p>\n<p>The numbers are large. At 20 N of tension the eccentricity is 2.8 percent, at 40 N it is 5.1, at 60 N it is 8.2, at 80 N it is 12.4, and at 100 N it is 17.1 percent. A line running at 100 N of tension cannot be centred by any die adjustment, because the offset is being imposed downstream of the die.<\/p>\n<p>The other causes are thermal and mechanical. The crosshead expands as it heats, and it expands unevenly, so a line that was centred cold is not centred hot. The capstan and the guide rollers wear and drift. And the die itself wears, usually more on one side, which produces a slow drift over months.<\/p>\n<p>The diagnostic order matters. Check the tension and the guides first, then the thermal centring, and adjust the die last. A die adjustment made to compensate for a tension problem is a die adjustment that will have to be undone when the tension is fixed.<\/p>\n<h2 id=\"measure\">How it is measured, and why the method matters<\/h2>\n<p>The reference method is a cross-section under a microscope or a profile projector. A slice is cut, mounted, polished and measured at several angular positions, and the result is a set of wall readings from which the minimum, the maximum and the eccentricity are calculated. It is accurate, it is slow, and it is a sample.<\/p>\n<p>The production method is capacitance. A conductor, an insulation and a screen form a capacitor, and the capacitance per unit length depends on the ratio of the diameters. An eccentric core changes the capacitance seen by a set of electrodes around the cable, and the variation as the cable rotates gives the eccentricity directly.<\/p>\n<p>The capacitance method is continuous, non-destructive and it runs at line speed, which makes it the only way to know what the whole drum looks like. It needs a calibration against a section, and it needs the cable to rotate or the electrodes to rotate, which is why it is usually built into the diameter gauge.<\/p>\n<p>X-ray is the third method and it is used for the cases the other two cannot see: a multi-core cable where the cores are not concentric with the sheath, or a cable with a metallic screen that blocks the capacitance measurement. It is a line tool and it costs accordingly.<\/p>\n<table>\n<thead>\n<tr>\n<th>Metoda<\/th>\n<th>Coverage<\/th>\n<th>Gives<\/th>\n<th>Limit<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Section, microscope<\/td>\n<td>Sample<\/td>\n<td>Absolute wall, eccentricity<\/td>\n<td>Powolny<\/td>\n<\/tr>\n<tr>\n<td>Profile projector<\/td>\n<td>Sample<\/td>\n<td>Wall, ovality<\/td>\n<td>Powolny<\/td>\n<\/tr>\n<tr>\n<td>Capacitance<\/td>\n<td>100%<\/td>\n<td>Eccentricity, online<\/td>\n<td>Needs calibration<\/td>\n<\/tr>\n<tr>\n<td>X-ray<\/td>\n<td>100%<\/td>\n<td>Core positions, sheath<\/td>\n<td>Koszt<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 1 \u2014 Concentricity measurement methods and what each can and cannot do<\/p>\n<h2 id=\"limits\">What the numbers should be<\/h2>\n<p>The limits are set by the standard and by the customer, and they are usually expressed as a minimum wall as a percentage of nominal rather than as an eccentricity. A common requirement is a minimum wall of 90 percent of nominal, which corresponds to about 10 percent eccentricity, and a tighter one is 95 percent.<\/p>\n<p>The tighter limit is expensive and it is worth understanding why. To hold 95 percent minimum wall the process has to hold about 5 percent eccentricity, which means the tension, the guides and the thermal centring all have to be controlled, and the line speed usually has to come down. That is a real cost and it should be attached to a real requirement.<\/p>\n<table>\n<thead>\n<tr>\n<th>Requirement<\/th>\n<th>Eccentricity<\/th>\n<th>Minimum wall<\/th>\n<th>Where used<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>General<\/td>\n<td>15%<\/td>\n<td>85% of nominal<\/td>\n<td>LV flexible cord<\/td>\n<\/tr>\n<tr>\n<td>Standard<\/td>\n<td>10%<\/td>\n<td>90% of nominal<\/td>\n<td>Building wire, control<\/td>\n<\/tr>\n<tr>\n<td>Tight<\/td>\n<td>5%<\/td>\n<td>95% of nominal<\/td>\n<td>MV power, instrumentation<\/td>\n<\/tr>\n<tr>\n<td>Precyzja<\/td>\n<td>3%<\/td>\n<td>97% of nominal<\/td>\n<td>Coaxial, RF<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p class=\"tablecap\">Table 2 \u2014 Typical concentricity requirements and what they cost<\/p>\n<div class=\"myth\">Myth: if the diameter is in tolerance the cable is good. Diameter is a sum of wall plus conductor plus screen, and a cable with a thick side and a thin side has the right diameter and the wrong wall. Diameter tells you nothing about where the conductor is.<\/div>\n<h2 id=\"fix\">Fixing a drifting line<\/h2>\n<p>Start with the tension. Measure it at the take-up and at the capstan, and check that the guide rollers are free and aligned. A roller that has worn a flat will pull the conductor sideways once per revolution, which shows up as a once-per-revolution variation in the capacitance signal and is often misread as a die problem.<\/p>\n<p>Then the thermal centring. Run the line to temperature with a scrap conductor, measure a section, and adjust the die hot. A die centred cold will be off by the differential expansion of the crosshead, which on a large head is several hundredths of a millimetre.<\/p>\n<p>Then the die itself. Check the die bore for wear and for a burr, and check that the die is seated squarely. A die that is not square produces an eccentricity that rotates with the die, which is a useful diagnostic: if the eccentricity follows the die and not the cable, the die is the problem.<\/p>\n<div class=\"faq\">\n<h3>Why is concentricity more important than diameter?<\/h3>\n<p>Because the insulation fails where it is thinnest. A cable with the correct diameter can have a thin side that halves its breakdown voltage, and the diameter measurement will not show it.<\/p>\n<h3>What causes a cable to go eccentric?<\/h3>\n<p>Most often take-up tension pulling the conductor to one side, then thermal expansion of the crosshead, then guide and capstan wear, then die wear. Check them in that order.<\/p>\n<h3>How is concentricity measured online?<\/h3>\n<p>By capacitance. The conductor, insulation and screen form a capacitor, and the variation as the cable rotates gives the eccentricity. It is continuous and non-destructive and it needs a section to calibrate against.<\/p>\n<h3>What eccentricity should a medium voltage cable hold?<\/h3>\n<p>About 5 percent, which gives a minimum wall of 95 percent of nominal. General purpose low voltage cord is usually allowed 15 percent.<\/p>\n<h3>Can a die adjustment fix a tension problem?<\/h3>\n<p>No. If the conductor is being pulled to one side after the die, adjusting the die only moves the problem. Fix the tension and then re-centre the die hot.<\/p>\n<h3>Why does eccentricity vary once per revolution?<\/h3>\n<p>A worn or misaligned guide roller, or a capstan with a flat. The period matches the rotation of the item that is causing it, which is how you identify which one.<\/p>\n<\/div>\n<div class=\"cta\">\n<p class=\"cta-h\">A cable that fails on wall or on partial discharge?<\/p>\n<p>Send the construction, the section and the line data. We will tell you whether it is tension, thermal centring or the die, and what the wall should be.<\/p>\n<p><a class=\"btn\" href=\"https:\/\/uplastech.com\/pl\/rozwiazania-w-zakresie-wytlaczania-tworzyw-sztucznych\/\">Ask about cable extrusion<\/a>\n<\/div>\n<div class=\"rel\">\n<h4>Continue reading<\/h4>\n<ul>\n<li><a href=\"https:\/\/uplastech.com\/pl\/wire-insulation-extrusion\/\">Wire Insulation Extrusion: Concentricity, Wall and the Spark Tester<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/cable-jacket-extrusion\/\">Cable Jacket Extrusion: Pressure Die, Drawdown and Surface Finish<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/crosslinked-pe-cable\/\">Crosslinked PE Cable: Silane, Peroxide and the Process Behind Each<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/cable-colour-coding\/\">Cable Colour Coding: Stripes, Rings and the Identification Nobody Reads<\/a><\/li>\n<li><a href=\"https:\/\/uplastech.com\/pl\/rozwiazania-w-zakresie-wytlaczania-tworzyw-sztucznych\/\">Plastic Extrusion Solutions: Profile, Sheet, Pipe and Tube<\/a><\/li>\n<\/ul>\n<\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>A cable does not fail because its diameter is wrong. It fails where the wall is thinnest, and 0.5 mm of wall holds 8 kV against 42 kV at 2.5 mm. Take-up tension is the cause people miss: 20 N gives 2.8 percent eccentricity, 100 N gives 17.<\/p>","protected":false},"author":1,"featured_media":8238,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[],"class_list":["post-8240","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-plastic-extrusion-process"],"meta_box":{"post-to-quiz_to":[]},"_links":{"self":[{"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/posts\/8240","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/comments?post=8240"}],"version-history":[{"count":1,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/posts\/8240\/revisions"}],"predecessor-version":[{"id":8665,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/posts\/8240\/revisions\/8665"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/media\/8238"}],"wp:attachment":[{"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/media?parent=8240"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/categories?post=8240"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/uplastech.com\/pl\/wp-json\/wp\/v2\/tags?post=8240"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}