CuW70 vs CuW80 for 145 kV, 245 kV and 420 kV GIS Arcing Contacts
Procurement engineers ask this in two forms. The first is "which grade do we need for 420 kV?" The second, from the supplier side, is "you have supplied 145 kV, why not 420?" Both rest on the same assumption: that voltage class maps to a copper tungsten grade. It does not, at least not directly. This article explains what actually drives the grade choice, what CuW70, CuW75 and CuW80 each give up and gain, and where the real limits sit when moving up the voltage classes.
What the Grades Are
The number is tungsten weight percent. The rest is copper. Industry-typical properties per GB/T 8320 for infiltrated material:
| Grade | Density (g/cm³) | Hardness (HB) | Conductivity (% IACS) |
|---|---|---|---|
| CuW70 | ≥ 13.8 | ≥ 175 | ≥ 42 |
| CuW75 | ≥ 14.5 | ≥ 195 | ≥ 38 |
| CuW80 | ≥ 15.2 | ≥ 220 | ≥ 34 |
These are minimums from the standard, not guaranteed values for any given lot. Certified figures are per lot, on request. ASTM B702 grades and RWMA classes cover the same ground with different designations; see GB/T 8320 vs ASTM B702 vs RWMA.
The direction is simple. More tungsten: harder, denser, more arc-erosion resistant, less conductive, more brittle, harder to machine and harder to join. Less tungsten: the reverse.
What an Arcing Contact Is Asked to Do
In an SF6 breaker or GIS interrupter, the main contacts carry the continuous current and part first. The arcing contacts, a fixed rod or tulip and a moving pin, stay closed a few milliseconds longer, then part and draw the arc while the gas flow extinguishes it. The arcing contact does not carry rated current for long. It exists to be eroded instead of the main contacts.
So the material property that matters most is erosion resistance per operation, which is a function of:
- Arc energy per operation, set by interrupting current, arc voltage and arcing time.
- Number of operations over the maintenance interval, which for a breaker on a utility line is dominated by a small number of short-circuit interruptions and a larger number of load and no-load operations.
- Gas flow and nozzle design, which decide how much of the arc root sits on the contact and for how long.
None of these is "voltage class". A 145 kV breaker rated 40 kA and a 420 kV breaker rated 50 kA see different arc energies, but so do two 145 kV breakers with different nozzle designs. The grade is chosen by the OEM against their own arc model and their type test results, not against a table.
Why Higher Voltage Classes Trend Toward CuW80
Having said that, there is a pattern. MV vacuum interrupters use CuCr, not CuW, and are outside this discussion. In SF6 breakers, CuW70 and CuW75 are common at 72.5 kV and 145 kV. At 245 kV, 420 kV and above, CuW80 dominates arcing contact tips and CuW75 or CuW80 shows up on the moving pin. The reasons:
Arc energy scales up. Higher system voltage means higher arc voltage and, with multi-break designs, longer arcing times per interrupter unit. More energy into the contact root per operation.
Contact geometry gets larger. A 420 kV arcing contact tip can be several times the mass of a 145 kV one, so the cost of the extra tungsten is a smaller share of the part cost, and the heavier part can absorb more heat.
The main-contact protection budget is tighter. On a 420 kV GIS, a maintenance outage is expensive. OEMs design the arcing contacts to last the full maintenance interval with margin, and they buy that margin with tungsten content.
Conductivity matters less at the arcing contact. The arcing contact carries current for milliseconds. The 8-point IACS drop from CuW70 to CuW80 is a small penalty there, and a large one on a main contact, which is why main contacts are not CuW.
None of this makes CuW80 mandatory at 420 kV or CuW70 wrong at 245 kV. It is a design outcome, not a rule.
Where CuW70 Is the Better Choice
CuW70 wins where the contact sees moderate arc energy and many operations, or where the part has thin sections and thermal shock is the failure mode:
- Load-break and disconnector arcing contacts with frequent operation and low interrupting current. Erosion per operation is small, thermal fatigue from cycling is the concern, and the higher copper content gives better crack resistance.
- Small-diameter moving pins where CuW80's brittleness risks fracture under mechanical impact at contact closing.
- Any part that is subsequently machined to fine features, because CuW70 machines with less tool wear and lower chipping risk at edges.
Where CuW80 Is the Better Choice
CuW80 wins where a single operation deposits a lot of energy and the contact must survive a limited number of them:
- Short-circuit arcing contact tips at 245 kV and above.
- Generator circuit breaker arcing contacts, where interrupting currents run to 100 kA and above, even though the voltage is only 20 to 30 kV. This is the clearest evidence that current and arc energy, not voltage, set the grade.
- Capacitive switching duty (C2 class per IEC 62271-100) where restrike resistance depends on a clean, erosion-resistant contact surface.
CuW75: The Compromise That Is Often the Right Answer
CuW75 sits between the two on every property and is the most common grade on 145 kV and 245 kV arcing contact rods from several OEM lineages. Its case is simple: enough erosion resistance for the arc energy at those classes, enough copper to friction weld and machine without the process sensitivity of CuW80. When an OEM has type-tested a design on CuW75, the drawing will say CuW75 and the question is closed.
The Real Limit Is Part Size, Not Voltage
Here is the point suppliers should be clear about. Nothing in the material or the process ties a CuW producer to a voltage class. The same infiltration furnace makes CuW80 for a 145 kV tip and for a 550 kV tip. What changes with voltage class is the part:
- Larger diameter and length. A 420 kV arcing contact tip may be 60 to 80 mm across; the moving pin assembly may be several hundred millimetres long with a CuCr shank.
- Press tonnage. The tungsten skeleton is pressed before sintering. A larger part needs a larger press, and the green density has to be uniform across the section or the infiltrated part has a density gradient.
- Furnace dimensions. Sintering and infiltration furnaces have a fixed hot zone. A part that does not fit, or a fixture load that unbalances the temperature profile, is a hard stop.
- Machining envelope. Long shanks with tight concentricity between the CuW tip and the CuCr rod need a lathe with the length capacity and the rigidity to hold tolerance on a hard-soft composite.
- Joining. Larger sections push the joint toward friction welding or EB, and the machine has to be sized for the part. See Cast-On vs Brazing vs Friction Welding.
When a supplier says "we have supplied 145 kV", the useful follow-up is not "why not 420 kV" but "what is your maximum part diameter, length and mass, and which joining processes do you run at that size". The voltage class on the assembly is the OEM's; the part envelope is the supplier's.
What the OEM Verifies at Higher Classes
Expect the qualification package to tighten as the voltage class rises, regardless of grade:
- Density per lot by Archimedes method, with the standard minimum as the floor and often an OEM minimum above it.
- Hardness and conductivity per lot.
- Metallographic section showing uniform tungsten distribution with no copper lakes above a stated size. Uniformity matters more on large sections because infiltration fronts can leave a gradient.
- Ultrasonic inspection of the tip-to-shank joint with an unbonded-area limit.
- First-article full dimensional report.
How to Specify Copper Tungsten Contacts on a Drawing lists the callouts that make these checks unambiguous.
Summary
CuW70, CuW75 and CuW80 trade conductivity and toughness for erosion resistance. Higher voltage classes trend toward CuW80 because arc energy per operation and part size both go up, but the grade is fixed by the OEM's design and type test, not by voltage class as such. Generator breakers at 24 kV use CuW80; load-break switches at 145 kV may use CuW70.
The supplier-side limit when moving from 145 kV to 420 kV parts is the part envelope: press, furnace, machining length and joining machine capacity. Ask for those numbers rather than for a voltage class.
Related Reading
- How to Choose Copper Tungsten Grade — Grade selection across MV and HV applications
- Copper Tungsten Properties Guide — Density, hardness, conductivity in depth
- GB/T 8320 vs ASTM B702 vs RWMA — Grade cross-reference
- CuW Arc Contacts — Made-to-drawing arcing contacts
- Copper Tungsten Series — Full CuW product range