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Isolation transformer vs site transformer

Updated: October 2026

You searched isolation transformer vs site transformer because a 110 V tool, a damp site and a 230 V repair job are easy to buy for with the same phrase. They are not the same device. A UK site transformer is a reduced-low-voltage supply, usually 110 V between lines and about 55 V from either line to earth, with the centre tap earthed. A 230 V isolation transformer in this amazon.co.uk set is a separating supply at mains voltage, and its secondary is meant to float. This page says which fault each one is for, and it will tell you to leave the isolator in the basket if the job is a building site. The three listings are on the [isolation transformer comparison](/hub/electrical-equipment/isolation-transformer). We compared published UK guidance with the saved listings. We did not lab-test either transformer.

You searched isolation transformer vs site transformer because a 110 V tool, a damp site and a 230 V repair job are easy to buy for with the same phrase. They are not the same device.

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
View offer
FUNTLY FUNTLY DB-20VA 20 VA chassis

Rank 3

FUNTLY FUNTLY DB-20VA 20 VA chassis
2.8/10about £22
View offer
On this page
  1. 1.Two different faults
  2. 2.What a UK site transformer does
  3. 3.What a 230 V isolation transformer does not do on site
  4. 4.When each one is the right buy

Two different faults

On a construction site the shock that the rules are written around is a fault to earth, often with a tool, a lead and a wet floor. The Health and Safety Executive tells construction work to use cordless tools or a 110 V supply centre-tapped to earth, so the voltage to earth does not exceed 55 V. Domestic 230 V leads are not that recommendation. The protection is a lower voltage to earth, plus automatic disconnection if a fault develops. Isolation, in the sense of two windings, is not the same thing as separation from earth. The IET notes that a reduced-low-voltage transformer can still be an isolating transformer whose secondary is earthed at the centre tap.

What a UK site transformer does

A site unit takes a 230 V primary and provides a 110 V secondary shown as 55 V–0–55 V. The centre point is earthed. Touching one line and earth is about 55 V, not 110 V, and not 230 V. NICEIC’s account of reduced low voltage on construction sites points to BS EN 61558-1 and BS EN 61558-2-23, with the secondary earthed at the midpoint. Yellow moulded connectors are the usual site pattern. It is the system the site tool was built for. The three listings kept in this comparison do not describe 110 V, a centre tap, a yellow site socket or part 2-23.
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 a 230 V isolation transformer does not do on site

A floating 230 V secondary does not give you 55 V to earth. A fault from one secondary line to a real earth may not return through the transformer at all, which is the separation idea, and a second fault, a bonded case or a lead that finds earth brings a full mains shock path back. The Schuko plug also does not fit a BS 1363 site board or a house socket. Using it on a building site fails the voltage, the earth bond and the connector. An autotransformer shares a winding. It does not provide the two-winding separation this comparison is about, and it is not a cheaper 110 V centre-tapped site unit.

When each one is the right buy

Buy 110 V centre-tap equipment, or use cordless tools, when the work is construction, demolition, or anywhere HSE’s site guidance applies: harsh, damp, or a temporary supply with hand tools. The transformer then needs part 2-23, a midpoint earth, and the disconnection arrangements the IET describes. None of B0GV3XRSCG, B0DKBZT4LV or B0CKBSHQR9 is that unit. The 230V isolation transformer buying guide is the next page only after you have decided separation is actually the job.

Common questions

Will a 230/230 isolation transformer run 110 V site tools?
No. The listings here state 220 V or 230 V out, not 110 V and not 55 V–0–55 V. A 110 V tool on a 230 V secondary is the wrong voltage, and a 230 V tool on a site transformer is the wrong voltage the other way. B0GV3XRSCG is 230 V to 230 V. The two chassis parts are 220 V to 220 V. None is a yellow site unit. Match the tool plate to the secondary plate before you look at VA.
Does a site transformer leave the secondary floating?
No. A UK reduced-low-voltage site transformer earths the centre tap. You still have two windings, so the circuits can be isolated from each other, and the secondary is not floating with respect to earth. The IET makes that distinction: isolation of the windings is not the same as separation from earth. Leaving the centre tap unearthed would abandon the 55 V-to-earth arrangement HSE describes.
Why do electricians talk about 55 V to earth?
Because a 110 V secondary with the centre tap earthed puts about 55 V between either line and earth. That is the touch voltage the IET and HSE are limiting, not the 110 V a meter shows between the two lines. It is why a site system is specified as centre-tapped to earth rather than as a floating 110 V winding. A floating 230 V pair does not produce that 55 V figure.
Is an autotransformer a cheaper site transformer?
No. An autotransformer does not give you separate windings, so it is not the isolating arrangement this page is comparing, and it is not a 110 V centre-tapped site unit under BS EN 61558-2-23. A lower price on an autotransformer page is a different product that has borrowed the word isolation. If you need 55 V to earth, you buy a site transformer that states the centre tap and the earth.
Who specifies a site temporary supply?
A competent electrician, using the construction rules rather than a retail title. HSE sets the cordless or 110 V centre-tap expectation. BS 7375 covers distribution on construction and demolition sites. NICEIC’s reduced-low-voltage account points to BS EN 61558-1 and part 2-23 with the midpoint earthed. None of the three amazon.co.uk listings in this set is offered as that supply.

Related pages

How to use an isolation transformer

Isolation transformer for a repair bench

Isolation transformer

Best isolation transformer UK from this Amazon set

230V isolation transformer buying guide for the UK

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)