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How to use an isolation transformer

Updated: October 2026

You have a portable isolation transformer and one appliance to run from it, and the tempting mistakes are to earth the output to be safe or to hang a four-way strip off the secondary. This page is how to use an isolation transformer when the job is electrical separation for a single plug-in load: read the plate, use one appliance, and leave the secondary floating. It is not a ranking, and it is not a method for a transformer screwed into the fixed wiring. The three listings are on the [isolation transformer comparison](/hub/electrical-equipment/isolation-transformer). The sequence follows the saved amazon.co.uk listings and public UK guidance. We did not energise a unit in a lab.

You have a portable isolation transformer and one appliance to run from it, and the tempting mistakes are to earth the output to be safe or to hang a four-way strip off the secondary.

Our top picks

Watts Control Watts Control 1.5 kVA cased isolator

Best overall

Watts Control Watts Control 1.5 kVA cased isolator
7.0/10about £149.99
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SACVXOKD SACVXOKD BK150VA 150 VA chassis

Best value

SACVXOKD SACVXOKD BK150VA 150 VA chassis
4.6/10about £41.99
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FUNTLY FUNTLY DB-20VA 20 VA chassis

Rank 3

FUNTLY FUNTLY DB-20VA 20 VA chassis
2.8/10about £22
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On this page
  1. 1.Read the plate before you energise it
  2. 2.One appliance, secondary left floating
  3. 3.What the wall RCD will not do
  4. 4.Where this sequence stops

Read the plate before you energise it

Before the plug goes into the wall, read the voltage, the continuous VA and any trip the plate or the listing actually states. UK public supply in Health and Safety Executive guidance is 230 V. A 220 V plate is not the same sentence. B0GV3XRSCG, the only cased listing in this set, states 230 V to 230 V, 1.5 kVA continuous, 3 kVA intermittent and a 10 A thermal-magnetic trip you reset by hand. It does not state a duty cycle. A load that stays on has to fit inside 1.5 kVA. At 230 V the 10 A trip is 2,300 VA, which is above the continuous rating and below the 3 kVA title.

One appliance, secondary left floating

Electrical separation in BS 7671 regulation group 413, as the IET describes it, is pictured for one item of current-using equipment. The protective effect is the missing earth-return path, not a lower voltage. The secondary of a separating supply stays floating. You do not add a link from either secondary line to the protective conductor and then still call the arrangement separated. Doing that ends the separation. One appliance means one plug in the output, not a four-way strip with a lamp, a radio and the device you meant to isolate. The IET’s picture is a single item. If you will clip earthed test gear onto the device, stop here and read the isolation transformer for electronics repair page, because a scope earth changes the reference this section is trying to keep.
Watts Control Watts Control 1.5 kVA cased isolator

Testsieger

Watts Control Watts Control 1.5 kVA cased isolator
7.0/10about £149.99
View offer

What the wall RCD will not do

A 30 mA RCD on the primary, which is the limit the Health and Safety Executive discusses for that device, protects against an earth fault on the supply side of the transformer. It does not replace electrical separation, and it does not make the secondary safe to touch. The secondary is still about 230 V between its own lines. If you contact both secondary conductors, the wall RCD is not in that circuit. Separation also does not replace proving a circuit dead before you work on it. You still isolate and prove dead. An isolation transformer is not a substitute for that test.

Where this sequence stops

This page stops at one plug-in appliance on a portable unit whose secondary you have left floating. It stops if you are about to connect the transformer to fixed wiring, bond the secondary because of PME, or supply more than one item. It stops for a medical IT system, an EV separating supply and a construction-site temporary supply. Those are specified installations. A site tool wants a 110 V centre-tapped transformer or a cordless tool, not this 230 V sequence.

Common questions

Do I earth the secondary?
Not if you want electrical separation. The IET’s account of regulation group 413 is one item, with the protective effect coming from the missing earth return. Bonding a secondary line to earth ends that arrangement and is a different installation. A note that tells a factory to earth the neutral when a solid neutral is required is not a step for this plug-in use. If you think the supply needs a bonded neutral, stop and get a competent person to design it.
Can I plug a four-way strip into the output?
No. A strip invites a second and a third appliance onto the same floating secondary, and each extra earth path or interconnected case weakens the single-item picture in regulation group 413. Use one plug for one appliance. On B0GV3XRSCG the two Schuko sockets are not an invitation to run two loads. Leave the spare socket empty. The chassis listings have no socket to extend in the first place.
What do I do if the trip will not reset?
Unplug the load first. If the 10 A trip on B0GV3XRSCG still will not stay in, or the case is hot, you leave the unit switched off and disconnected. Do not hold the reset in or bypass it. The listing does not describe a user service procedure, and there are no reviews to tell you this is normal. A trip that only opens when a particular appliance is connected means that appliance is the thing to investigate, still with the supply proved dead.
Can I follow this page for a transformer that is screwed to the fixed wiring?
No. Fixed wiring, a medical IT transformer, an EV separating supply under regulation 722.413.1.2, and a site temporary supply are design jobs. This sequence is for a portable unit and one plug-in appliance. If the transformer is part of the installation, or you intend to bond the secondary, you need a competent electrician. Do not adapt these steps to a consumer unit or a hard-wired bench.
Which of the three listings is even a plug-in unit?
Only B0GV3XRSCG states a plug, sockets and a trip, and that plug is 16 A Schuko, not BS 1363. B0DKBZT4LV and B0CKBSHQR9 state no lead and no trip. You cannot follow a socket sequence with a bare chassis part. If the unit in your hand has no continuous VA on the plate, you do not invent one from the 3 kVA style of title used on the cased listing.

Related pages

230V isolation transformer buying guide

Isolation transformer vs site transformer

Isolation transformer

Best isolation transformer UK from this Amazon set

Isolation transformer for electronics repair limits

Sources

Public UK sources from the research digest. Shelf prices are the amazon.co.uk figures in the display file. We did not treat trade-catalogue prices as amazon.co.uk prices.

  1. HSE: electricity on construction sites (retrieved 22 October 2026)
  2. HSE: electrical equipment (retrieved 22 October 2026)
  3. Electrical Equipment (Safety) Regulations 2016, regulation 35 (retrieved 22 October 2026)
  4. GOV.UK: Electrical Equipment (Safety) Regulations 2016 (retrieved 22 October 2026)
  5. BSI: BS EN IEC 61558-2-4:2025 (retrieved 22 October 2026)
  6. IET Wiring Matters: separation and isolation (retrieved 22 October 2026)
  7. Carroll & Meynell: 230 V to 230 V isolation transformer duty (retrieved 22 October 2026)
  8. ABL: isolation transformer sizing (retrieved 22 October 2026)