Cable lugs for transformers and UPS

Duty current and fault current

A transformer secondary or a UPS output can deliver a current far above its normal rating for a short time. The lug has to carry the duty current without overheating and the fault current without melting, annealing or losing its grip before the protection operates.

The duty current sets the conductor size and the steady-state temperature. The fault current sets the short-time withstand. The two are checked separately. A lug that is generous for the duty current may still be too small for the fault level if the protection is slow.

The fault duration comes from the protection settings, not from a generic assumption. A fuse may clear in a few milliseconds at a high fault level, while a breaker with a deliberate delay may take several hundred milliseconds. The lug and the conductor have to be checked against the actual clearing time.

The terminal interface

Transformer and UPS terminals are usually a flat pad with one or more tapped holes or a stud. The lug palm has to match the pad, the hole spacing and the bolt size. A palm that is too small concentrates the clamp load and may not cover the pad; a palm that is too large can foul the phase barrier or reduce the clearance.

The bolt and washer stack are part of the current path. A plated bolt, a flat washer and a spring washer or Belleville washer each do a job. A plain washer under a bolt head spreads the load, while a spring element maintains clamp force as the joint relaxes. Mixing hardware from different sources without checking the material and plating can create a galvanic pair.

Many transformer terminals are aluminium. A copper lug on an aluminium pad is a bimetallic joint and needs a transition or a specified interface treatment. The same applies to a copper conductor in an aluminium lug. The joint has to be designed as a pair, not selected from a table of lug sizes.

Thermal cycling and clamp force

A transformer or UPS runs warm and cycles with load. Each cycle expands and contracts the copper, the aluminium and the steel bolt by different amounts. The clamp force can fall as the materials relax, and a joint that was correct at commissioning may be loose a year later.

A torque value alone is not enough. The torque has to be applied to a clean, dry or correctly lubricated thread, with the specified washer stack, and it has to be checked after the first thermal cycle. A marking on the bolt shows whether it has moved. A re-torque schedule is a practical control for a joint that cannot be monitored continuously.

The plating affects relaxation. Tin creeps at high temperature, so a tin plated joint may lose force faster than a silver or nickel plated one. The plating choice and the clamp hardware should be considered together, especially on a joint that runs hot.

Short-time withstand

The short-time withstand rating is the current a joint can carry for a stated time without exceeding a temperature limit. It is usually expressed with the duration, because the same joint has a different rating for one second and for three seconds. The conductor, the lug and the terminal all have to survive.

The adiabatic check is a useful first pass. The minimum conductor area is the square root of the current squared times the time, divided by a material factor. For illustration, a 20 kA fault for 0.2 seconds with a factor of 143 for XLPE insulated copper gives a minimum area of about 63 mm squared. A 100 mm squared conductor would pass that check, but the lug, the joint resistance and the terminal would still need to be verified.

The check is only a first pass because it assumes no heat leaves the conductor during the fault. A real joint has contact resistance and a terminal that may be the hottest point. The test that matters is a short-time current test on the actual assembly, or a type test on an equivalent assembly from the same maker.

Clearance, creepage and insulation

The lug sits inside an assembly with its own clearance and creepage requirements. A palm that is correct on its own can reduce the distance to a phase barrier or an earthed enclosure. The cable insulation, the heat shrink and any phase barrier all count in the distance.

The assembly standard sets the required distances for the pollution degree, the altitude and the insulation coordination. A larger lug may need a larger enclosure or a different barrier. A smaller lug may fit but run hotter. The layout has to be checked with the cable fitted, because a bent cable can reduce a clearance that looked adequate on the drawing.

Insulation on the lug itself is sometimes used to improve the electrical clearance or to protect against accidental contact. A sleeve or boot has to be rated for the temperature and the voltage, and it must not trap heat around the barrel. The insulated version of a lug is a different part, not the same part with a cover added.

Plating and the connection atmosphere

Tin is the usual finish for a transformer or UPS lug in a dry, protected enclosure. It is cheap, solderable and adequate at the temperatures inside most equipment. Silver is used where contact resistance and current density are critical, and nickel where the temperature is higher or the atmosphere is more aggressive.

The plating has to suit the terminal. A tin plated lug on a silver plated pad is a mixed pair, though a less aggressive one than copper on aluminium. A silver plated lug on a tin plated pad can tarnish and increase resistance. Where the pad finish is fixed, match the lug finish or document the pair and the joint treatment.

The plating thickness is as important as the metal. A thin flash can wear through during installation, especially on a large palm that is pulled into position. A thicker deposit costs more but gives more margin for handling and for the life of the joint.

A specification sequence

Start with the duty current and the ambient. Choose the conductor and the lug size. Then check the fault current and the clearing time. After that, match the palm to the terminal and choose the clamp hardware. Finally, check the clearance and the plating, and define the tests and records.

The order matters because a change late in the sequence can reopen an earlier decision. A larger fault current may force a larger conductor, which changes the lug barrel and the palm. A larger palm may reduce the clearance and force a change in the enclosure. Working through the sequence once is faster than iterating on a finished layout.

Records that support the installation

A transformer or UPS joint is often a critical connection in a building or a process. The records should let a later engineer see what was installed: the lug part number, the conductor, the plating, the torque, the washer stack and the test result. A photograph of the joint before the cover goes on is a cheap and useful addition.

If the assembly is type tested, keep the test report with the assembly drawing. If it is not, the project may need a site test. A contact resistance measurement and a thermal image at load are practical checks that do not require a laboratory.

The record is also the basis for maintenance. A joint that is re-torqued without a note, or a lug that is replaced with a different plating, loses the traceability that the original design depended on. Keep the specification with the equipment file, not only with the supplier.