Copper Tungsten Contacts for Mining and Explosion-Proof Switchgear: What Changes
Underground mining switchgear is a different animal from a utility substation breaker, and the contact materials inside it are asked different questions. The voltage is lower, the switching is far more frequent, the environment is worse, and the enclosure is certified as a whole for explosive atmospheres. This article covers where copper tungsten is used in mining and explosion-proof (Ex) switchgear, what changes compared with utility duty, and what a buyer sourcing replacement or OEM contacts for this equipment should specify.
Where CuW Sits in Mining Switchgear
A typical underground distribution unit for a coal or hard-rock mine is a flameproof combination switch or a flameproof vacuum starter: a certified Ex d enclosure containing a vacuum contactor or vacuum circuit breaker, an isolator (disconnect switch), fuses or protection relays, and cable glands. Voltages run 1,140 V, 3.3 kV, 6 kV or 10 kV depending on the region and the mine. Copper tungsten appears in three places:
Isolator arcing contacts. The isolator is nominally off-load, but mine isolators are specified to make onto a fault, and some are used as the switching device on smaller units. The main contacts are copper or silver-plated copper; the arcing tips or arcing horns that take the make and the occasional break are CuW, typically CuW70 or CuW75.
Earthing switch contacts. Same logic. A mine earthing switch may be closed onto a live circuit by mistake, and the contact has to survive it.
Air-break contactor and switch contacts in older or lower-voltage units, where the switching element is not vacuum. Less common in new equipment, still common in the installed base and the spares market.
Inside the vacuum interrupter itself the contacts are copper chromium, not CuW, and they are not a separately sourced part. Vacuum contactors for motor duty often use silver tungsten carbide. CuW's territory in mining gear is the air-insulated arcing contact outside the vacuum bottle.
What Changes Compared With Utility Switchgear
Duty cycle
A 145 kV transmission breaker may operate a few hundred times a year and interrupt a short circuit a handful of times in its life. A mine contactor feeding a conveyor or a shearer starts and stops many times per shift. The isolator behind it is operated for every maintenance isolation. Contact erosion is a per-operation cost multiplied by a large number, so wear rate under moderate arcs matters more than survival of a single severe arc.
This pushes the grade choice toward the copper-rich end. CuW70 gives better thermal shock resistance and lower contact resistance across many operations; CuW80's extra erosion resistance per severe arc is worth less when the arcs are mostly small. Where a drawing calls for CuW75, it is usually a compromise for isolators that must also make onto a fault. See CuW70 vs CuW80 for the property trade-offs.
Environment
Coal dust, rock dust, high humidity, mine water with dissolved salts, methane in the atmosphere, vibration from nearby machinery. The enclosure keeps the explosive atmosphere out of the arc, but the air inside the enclosure is mine air. Consequences for the contact:
- Silver plating on the current-carrying faces is close to universal, and thicker than in utility gear. Copper oxidises quickly in humid, sulphurous air and contact resistance climbs.
- The arcing face is unplated as usual, but the transition between plated and unplated zones has to be masked cleanly, because a silver edge under the arc root erodes fast and lifts.
- Corrosion at the CuW-to-copper joint is a known failure mode if the braze fillet is exposed. Some drawings specify a plated-over or sealed joint line.
Fault levels
A mine distribution transformer is close to the switchgear, so prospective fault current at the isolator can be high relative to the rated current. The making capacity requirement on the arcing contact is therefore significant even though the voltage is low. This is why mine isolators carry CuW tips at all; a utility disconnector at the same voltage often does not.
Space and mass
Flameproof enclosures are heavy and expensive per litre of internal volume. Contact assemblies are compact, and the CuW tip is often small: a few grams brazed to a copper finger. Small tips mean thin sections, which is another reason CuW70 and CuW75 are favoured over the more brittle CuW80.
Certification is at enclosure level
An Ex d flameproof enclosure is certified under IEC 60079-0 and IEC 60079-1 (and regionally under ATEX, IECEx, or national mining approval schemes). The certificate covers the enclosure and its flameproof joints, and the internal equipment as documented by the certificate holder. It does not certify the contact material. This is the same principle as type testing in utility switchgear: the apparatus is certified, the component is specified. See Why Type Testing Is Done at Apparatus Level.
What it means in practice: a replacement contact must match the drawing the certificate holder documented. The material, geometry and plating are part of the documented internal equipment. A contact that is dimensionally identical but a different grade or plating is a change to certified equipment, and whether that change is acceptable is the certificate holder's decision, not the supplier's or the mine's.
Sourcing Contacts for Mining Switchgear
Three buyer situations, three approaches.
OEM producing new units. Drawing exists, grade and plating are decided, the question is supplier qualification. Specify infiltrated CuW with density, hardness and conductivity minimums, plating thickness by XRF, and joint acceptance by ultrasonic test or bond strength. How to Specify Copper Tungsten Contacts on a Drawing has the full list.
OEM or service company supplying spares for their own certified equipment. Same as above, with attention to drawing revision control. The contact in a 15-year-old unit may be on an older drawing revision than the current production part, and both may be valid. Confirm which revision the certificate documentation references.
Mine or contractor sourcing replacements without OEM drawings. This is common, especially for older units and units whose OEM no longer supports them. The route is reverse engineering from a sample: measure the part, identify the grade by density and hardness, identify the plating by XRF, section a worn sample to establish the joint method, and produce a drawing. Two cautions. First, the sample is worn; the arcing face dimensions on the drawing should be reconstructed from the unworn portions and from the mating part, not copied from the eroded tip. Second, the certificate point above applies. A replacement made to a reverse-engineered drawing is a part the certificate holder has not documented. For a mine operating under a regulatory regime, that decision sits with whoever holds responsibility for the equipment's Ex status, and it should be made knowingly.
What We Ask For
When a mining switchgear enquiry arrives, the questions we put before quoting are:
- Drawing or sample. If sample, is the mating part available for reference?
- Grade, or if unknown, may we determine it from the sample by density and hardness?
- Plating: material, thickness and masked zones.
- Joint method on the existing part, and whether the same method is required or an equivalent is acceptable.
- Quantity and whether this is a recurring spare or a one-off.
- Whether the part goes into certified Ex equipment, and who holds the certificate.
The sixth question is not a formality. It decides whether we are supplying to an OEM drawing, to a documented spare, or to a reverse-engineered part where the buyer carries the certification decision.
Summary
In mining and explosion-proof switchgear, copper tungsten is used on isolator and earthing switch arcing contacts and in older air-break switching devices, mostly as small tips brazed to plated copper fingers. Frequent operation, harsh atmosphere and thin sections favour CuW70 and CuW75 over CuW80. Silver plating on current-carrying faces is thicker and more critical than in utility gear.
The enclosure's Ex certificate covers the documented internal equipment, not the contact material. Replacement contacts should match the certificate holder's drawing revision, and reverse-engineered spares should be sourced with the certification implications understood by the buyer.
Related Reading
- CuW70 vs CuW80 for 145 kV, 245 kV and 420 kV — Grade trade-offs
- Why Type Testing Is Done at Apparatus Level — Certification versus component specification
- How to Specify Copper Tungsten Contacts on a Drawing — Drawing callouts
- Static vs Moving Arc Contacts — Contact roles in switching devices
- CuW Contact Finger — CuW-tipped fingers made to drawing