What are plug-in isolation transformers?
Series isolation transformers supplied with moulded plug-and-receptacle connectors on their leads, so they are fitted by plugging in rather than by making joints in the pit.
A plug-in isolation transformer arrives with its leads already ending in moulded plug-and-receptacle connectors. It is fitted by plugging into the loop cable and the fixture lead, not by cutting cable and making joints in the pit.
1The joints are made in the factory
The connector bodies, contacts and seals are moulded onto the leads under controlled conditions. That gives a consistent joint every time, without relying on a jointing kit used in a wet pit.
2A quicker, cleaner change
A unit with three connections, two on the primary and one on the secondary, can be changed by unplugging and plugging in rather than cutting and re-jointing. Less work in the pit helps, but only on a circuit that is isolated, locked off and proven dead first.
3The plugs become the joints to look after
The primary connector carries the full loop current, so a loose, corroded or wet plug can break the whole circuit. The secondary plug affects one fitting. Contacts must be clean and fully mated, and seals and boots intact. See Primary Connector and Secondary Connector.
4Match the whole unit
Plug-in describes how the unit connects, not what it does. A replacement must still have the same current ratio and a suitable wattage rating, and its connectors must be compatible with the cable and fixture leads already there.
| Point | Plug-in unit | Field-jointed unit |
|---|---|---|
| Connections made | Factory-moulded plugs | Joints made on site |
| Fitting | Plug into loop cable and fixture lead | Cut, joint and seal in the pit |
| Replacement | Unplug, swap, plug in | Cut out, re-joint, re-seal |
| What to watch | Right type, clean contacts, full mating, seals | Quality of every joint made |
Frequently Asked Questions
Q1What is a plug-in isolation transformer?
It is a series isolation transformer supplied with its primary and secondary leads already terminated in moulded plug-and-receptacle connectors. It is fitted by plugging into the loop cable and the fixture lead, rather than by making joints in the pit.
Q2How is it different from a field-jointed transformer?
A field-jointed unit has bare or plain leads, and the joints to the primary cable and the fixture lead are made on site with a jointing kit. A plug-in unit moves most of that work to the factory, so there is less jointing to do in the pit.
Q3Can a plug-in transformer be unplugged while the circuit is energised?
No. The primary is part of the series loop, so opening a primary connector on a live circuit breaks the loop. Isolate the CCR, lock off, prove the circuit dead and make it safe before separating any connector.
Q4Is replacement faster with plug-in units?
Usually, because the joints are plugged rather than cut and re-made. The saving depends on the unit and the site, and the safety, testing and recording steps for a replacement are the same as for any transformer.
Q5Do plug-in transformers change the rating or ratio?
No. Plug-in describes how the unit connects, not what it does. The current ratio and wattage rating still come from the nameplate and must suit the circuit and the fixture.
Q6What should be checked on the connectors?
That the type matches, the contacts are clean and fully mated, and the seals and boots are in good condition. A loose, corroded or wet connector is a common cause of a dim fitting or a circuit fault, so the plugs need the same care as any joint.
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