The connection between a cable and the LV bushing stud carries hundreds of amps. A small rise in contact resistance is enough to release tens of watts of heat at a single point – and that heat goes straight into the stud, the bushing insulator and its seal. Below: where hot clamps come from, how to avoid them during installation and how to catch them before they cause a failure.

How much heat a bad contact produces

The power lost in a joint is P = I² × R. On a 630 kVA transformer the LV phase current is about 900 A. At that current every extra 0.1 mΩ of contact resistance means about 81 W of continuous heating in a single clamp. A hot contact oxidises faster, its resistance keeps rising and the process feeds itself.

The most common causes of clamp overheating

Too little clamping force

Clamping screws tightened “by feel” or never checked after commissioning. Conductors – aluminium ones in particular – settle under pressure and the clamping force drops over time.

Too much clamping force

Crushed strands, stripped threads, a cracked body. Tighten the screws with a torque wrench, gradually and alternately – not with brute force and not with an extension on the wrench.

Dirty or oxidised contact

Oxide on an aluminium conductor, a dirty stud, paint residue. Clean the conducting surfaces immediately before installation. TOC clamps accept aluminium and copper conductors – the tin-plated body limits corrosion at the contact with aluminium, but the conductor still has to be cleaned.

Poorly prepared conductor

Stripped too short (insulation under the screw) or too long (bare conductor sticking out of the socket), a cross-section outside the socket range, or a conductor shape the clamp is not designed for.

Cables hanging on the clamp

Heavy LV cables that are not supported bend and twist the stud. This increases the risk of a loose joint, bushing damage and oil leaking past the seal.

Higher load than the clamps were selected for

After an installation is extended, the current is often higher than the one the connections were chosen for – and it is not only the clamp that heats up. More on running a transformer near its limit in our article on transformer ageing.

Installing a transformer clamp step by step

1. Inspect the bushing: cracks in the porcelain, oil traces, the condition of the stud thread.
2. Clean the stud and the clamp sockets of dirt and paint residue.
3. Screw the clamp onto the stud, align the sockets with the cable route and lock it with the fixing screws (TOC-1 – two screws on the foot, TOC-2 – screws on the side of the slotted stem).
4. Prepare the conductors: strip them to the depth of the socket, clean aluminium conductors of oxide with a wire brush and – if the cable manufacturer recommends it – apply contact paste.
5. Insert the conductors fully and tighten the clamping screws with a torque wrench – gradually and alternately.
6. Support and fix the cables so that they do not load the clamp or the stud.
7. Fit the insulating cover over the clamp.
8. After commissioning, once the load has stabilised, carry out a thermographic check and re-check the clamping force in line with the substation's operating instructions.

A set of TOC-1 clamps on the LV bushings – sockets aligned with the cable route.

Thermographic inspection – how to read the result

Thermography finds a bad contact without switching the transformer off. For the result to mean something:

• measure under load – the closer to the substation's typical load, the clearer the difference,
• compare the same point on all three phases; the difference between phases at similar current tells you more than the temperature alone,
• watch out for shiny surfaces: tin-plated brass has low emissivity and the camera under-reads its temperature – take readings from the cable insulation right next to the clamp or from a matt marker.

A useful reference are the criteria of the US standard ANSI/NETA (table 100.18), used by many inspection companies:

If the grid operator or the plant has its own operating instructions with different thresholds, those apply.

LV connection inspection checklist

• discolouration, deposits or melted insulation next to the clamp,
• oil traces under the LV bushings,
• condition of the clamp covers, traces of birds and rodents,
• cable fixing and support,
• comparative thermogram of the three phases and the neutral,
• screw tightness check – only when de-energised and earthed, as per instructions.

Regular inspection costs a fraction of a repair – see what a professional transformer overhaul looks like. If a clamp needs replacing, start with choosing the transformer clamp for the rating – ready-made sets are in our transformer clamps category.

Energeks transformer clamps – in stock in Poland

TOC-1 – horizontalM12 / M20 / M30, 250–1000 A, 2 × 300 mm²TOC-2 – compactsame connection capacity, smaller footprint

Tell us the transformer rating and cable sizes – we send the quote the same day.Request a quote