A “transformer clamp” is in practice several different products. On the LV side they connect cables or busbars to the bushing studs; on the MV side they connect conductors to the high-voltage bushings, sometimes together with a surge arrester. Below is an overview of the types you will find in MV/LV substations and tips on when to use which.

Where a transformer clamp works

The clamp screws onto the threaded stud of the bushing insulator and is locked with one or more screws. On the other side it has sockets for the cable conductors or a flat face for a busbar. Grid operators' standards – for example that of the Polish DSO TAURON Dystrybucja for MV/LV transformers – provide two LV variants: clamps for conductors or cables and clamps for a busbar connection, in both cases without crimped lugs, in a set of three phase clamps and one neutral clamp.

Horizontal cable clamps (LV)

The most common type in substations with transformers up to 630 kVA. They take the cables out horizontally from the stud and accept two main and two auxiliary conductors per phase – aluminium or copper. At Energeks this is the TOC series: M12, M20 or M30 thread, 250, 630 or 1000 A, main conductors 2 × 95–240 mm² (RE, RM, SM) or 2 × 300 mm² RMC, auxiliary 2 × 2.5–50 mm².

TOC-1 – with an offset foot

Two locking screws on the foot under the head, sockets higher above the cover. Convenient with thick cables – it is our standard for MarkoEco2 transformers. TOC-1 clamp page.

TOC-2 – compact

Sockets in a single compact head, a slotted stem and locking screws on the side. It takes less room on the cover, so it works well where the LV bushings are close together. TOC-2 clamp page.

Neutral clamps with earthing

The fourth clamp in the set sits on the neutral bushing. Besides the cable sockets it has a stud (or two) for the earthing conductor, N/PE or an earthing strip. In the TOC series these are the /N versions: DP 5100N–5300N (TOC-1) and DP 6100N–6300N (TOC-2). Some operators require that a 25 × 4 mm earthing strip can be connected to the neutral clamp (e.g. Energa-Operator) – the /N versions of the TOC series allow this.

TOC-1/N – version with an extra stud for the neutral or earthing conductor.

Busbar clamps (LV)

When the transformer is connected to the LV switchgear by busbars, clamps with a flat face for the busbar are used instead of cable sockets. Forged MK busbar clamps screw onto the stud and are locked with one screw (MK 1) or two (MK 2, MK 5). They take busbars up to 40 mm wide (MK 1, MK 2) or up to 100 mm (MK 5), on studs from M12 to M30. Energeks supplies MK 1, MK 2 and MK 5 from stock in Poland.

MV bushing clamps

On the high-voltage side the clamp connects the MV conductor to the bushing of the distribution transformer. ZGU clamps (M12 connection thread, 70 mm² conductor) are made of tin-plated brass with stainless steel screws and are fitted with a single wrench. The ZGU-OP version allows an MV surge arrester to be mounted directly on the bushing stud, without additional parts. The design limits torsional forces transferred to the internal parts of the transformer. ZGU and ZGU-OP are available from stock.

Clamp covers

An insulating cover fitted over the clamp protects against accidental contact and short circuits caused by birds and rodents. The TAURON Dystrybucja standard requires LV clamp covers made of a self-extinguishing, flexible material. The OZNN cover is made of flexible, self-extinguishing rubber in three sizes – M12, M20 and M30 – and fits TOC-1, TOC-2 and MK 1 clamps.

Comparison at a glance

How to choose

• cables to the LV switchgear → TOC-1 or TOC-2 cable clamp (thread and current as described in how to choose a transformer clamp),
• busbars → MK busbar clamp matching the busbar width,
• neutral → /N version,
• MV side with a surge arrester on the bushing → ZGU-OP,
• substation exposed to birds and rodents → covers on all LV clamps.

All types in one place: Energeks transformer clamps.

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