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Isolation transformer for electronics repair

Updated: October 2026

You have a live chassis on the bench, a scope with an earthed clip, and a listing that seems to promise you can probe it once the yellow or black box is in circuit. An isolation transformer for electronics repair changes one fault path for one device under test. It does not make the secondary safe to touch, and it does not survive an earthed instrument lead as if nothing had been connected. This page follows that limit for a competent person. It does not rank the three amazon.co.uk listings, and it does not turn HSE’s workplace rules into features those listings never state. The rows are on the [isolation transformer comparison](/hub/electrical-equipment/isolation-transformer). The limits come from HSE guidance and the saved listings. We did not probe a live chassis.

You have a live chassis on the bench, a scope with an earthed clip, and a listing that seems to promise you can probe it once the yellow or black box is in circuit.

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
View offer
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.What the floating supply is for
  2. 2.What HSE expects around the bench
  3. 3.Class I gear and a fault that is already there
  4. 4.Which listings do not close the gap

What the floating supply is for

Electrical separation, in the IET’s account of BS 7671 regulation group 413, is pictured for one item of current-using equipment. The protective effect is the missing return path to earth, not a reduced voltage. Between the two secondary lines you still have about 230 V. A single fault from one line to earth may not complete a circuit through the supply. A second fault, or a path you add yourself, can complete it. Clipping the earth of an oscilloscope, or any other earthed instrument, onto the unit references the circuit to the instrument’s earth. You have reintroduced the path the transformer was there to omit.

What HSE expects around the bench

HSE’s leaflet INDG354, Safety in electrical testing at work, describes an earth-free test area. Where practicable there is one isolating transformer per bench. The supply comes from a single socket marked for live equipment under test, and there is no fixed wiring on the earth terminal of that outlet. The transformer is to BS 61558. They are not a checklist you can tick because a marketplace title says isolation. Defeating the earth on a Class I appliance to make the hack work is outside this page. If you cannot provide the earth-free area and the single socket, the transformer alone does not finish the precaution.
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

Class I gear and a fault that is already there

INDG354 says a pre-existing earth fault on Class I equipment must be found before the equipment is made live. You do not discover that fault by energising the chassis through the transformer and waiting. The protective conductor has to be proved, the fault cleared, and the conductor proved again afterwards. Skipping that check because the secondary is floating leaves a fault in place for the moment an earthed lead, a second fault or a bonded case restores the path. Nothing on this page tells you to lift the earth pin of a Class I plug as a repair method.

Which listings do not close the gap

None of the three amazon.co.uk listings is described as a test-bench supply. B0GV3XRSCG at £149.99 cites BS EN 61558 with no part number, so it does not state the BS 61558 conformity HSE names, and it does not state part 2-4 either. It has two Schuko sockets rather than one marked BS 1363 socket with the earth terminal left unwired. A bench load still has to fit the continuous figure, and the Schuko plug still does not fit a UK 13 A socket. Choosing a non-bench job belongs on the best isolation transformer page, still without a star-rating crown.

Common questions

Can I clip an earthed oscilloscope onto a unit that is on the isolated supply?
Only if you accept that the scope earth re-references the device. The floating secondary’s missing earth return is what regulation group 413 is relying on for one item. An earthed probe clip puts that reference back. The secondary is still about 230 V between its lines, and the setup is not safe to touch because the transformer is in circuit. If the measurement needs an earthed instrument, you need a method that assumes the earth is present, not this floating-supply picture.
Does HSE expect one transformer per bench?
Where practicable, yes. INDG354 describes one isolating transformer per test bench, feeding a single socket marked for live equipment under test, in an earth-free area, with no fixed wiring on that socket’s earth terminal, and a transformer to BS 61558. That is workplace guidance. It is not a feature claimed by B0GV3XRSCG, B0DKBZT4LV or B0CKBSHQR9. If you cannot arrange the area and the single socket, the box on its own does not meet the leaflet.
Should the socket earth pin be connected?
HSE’s description of that test socket says there is no fixed wiring on the earth terminal of the outlet. That is a designed test outlet in an earth-free area, not an instruction to cut the earth out of a household BS 1363 socket or to run a Class I appliance with the protective conductor defeated. Building that outlet is competent-person work. The Schuko sockets on B0GV3XRSCG are not described as that test outlet.
Is a 20 VA or 150 VA chassis part a bench supply?
No. B0CKBSHQR9 states 20 VA and dimensions of 1 x 1 x 1 cm, with no standard and no socket. B0DKBZT4LV states 150 VA and 0.68 A, again with no socket and no standard. HSE’s bench supply is a BS 61558 transformer and a single marked socket. A bare chassis part does not state either. A load above 0.68 A is already beyond the 150 VA listing, before the enclosure question starts.
What still has to be proved dead?
The circuit you are about to touch. An isolation transformer does not prove a capacitor discharged, a soldering iron isolated, or a chassis dead. You isolate and prove dead before contact, including after you have used the floating supply. A pre-existing earth fault on Class I equipment has to be found before the equipment is made live, and the protective conductor proved again afterwards. The wall RCD does not replace either check.

Related pages

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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)