Electrical separation (§413.3) — an isolating transformer as a protective measure without earthing

The §411 earthing guide covers the most common protective measure: a protective conductor combined with automatic disconnection of supply. The §414 SELV/PELV/FELV guide covers a second measure: a reduced voltage that limits shock risk. §413.3 describes a third, less well-known measure: electrical separation — where the voltage is not reduced, but the circuit itself is kept entirely isolated from earth.

The principle: no closed fault-current loop

In a normal, earthed installation, a single fault — a live conductor accidentally touching an earthed metal part — creates a closed loop: the fault current flows back to the source via the earthing system, and it is precisely that current that trips the protective device (or, without protection, creates a touch voltage that can be dangerous).

With electrical separation, the secondary circuit of an isolating transformer (with reinforced or double insulation between primary and secondary winding) is kept entirely isolated from earth. If a single live conductor of such a circuit touches an earthed part, no closed current loop is created — there is no connection between the secondary circuit and earth to close the loop. A single fault therefore has no dangerous consequence on its own.

Note: this is a fundamentally different mechanism from SELV/PELV. With SELV/PELV, the reduced voltage is the safety factor — a touch voltage of, say, 25 V is generally not dangerous, even though a closed loop does exist. With electrical separation, the absence of a closed loop is the safety factor — the voltage on the secondary side can, unlike SELV/PELV, simply remain the normal operating voltage (e.g. 230 V).

Why the basic rule is one item of equipment per circuit

The classic implementation of electrical separation supplies a single item of equipment from one secondary winding. The reason: as soon as two or more items of equipment are connected to the same separated secondary, a double fault (each item separately developing a fault to its own enclosure) could still produce a dangerous touch voltage between the two enclosures, especially if those enclosures are accessible to each other or interconnected. Supplying multiple items of equipment from a single isolating transformer therefore requires additional conditions (such as an unearthed supplementary bonding connection between all exposed-conductive-parts of those items, and monitoring of the insulation condition) — this is an exception to the main rule, not a replacement for it.

Where this is applied in practice

Portable isolating transformers for hand tools on construction sites are a well-known practical application — see also the §704 construction sites guide for the broader context of protective measures on a construction site.
Shaver socket-outlets in bathrooms (with a built-in small isolating transformer) are a fixed application of the same principle: no reduced voltage, but a galvanically separated circuit without an earth connection.

Practical relevance

When assessing an installation with an isolating transformer, first establish which principle is actually being applied: is this electrical separation (§413.3, normal voltage, no earthing of the secondary circuit) or SELV/PELV (§414, reduced voltage)? Next, check whether the "one item of equipment per secondary winding" starting point is being respected, and whether the secondary side has not accidentally been connected to earth after all — because that would defeat the entire protective measure.

Common mistakes

Confusing electrical separation with SELV/PELV — assuming an isolating transformer always delivers a reduced voltage, while the secondary voltage with electrical separation can simply remain 230 V.
Earthing one conductor of the "separated" secondary circuit after all — this removes exactly the mechanism (no closed loop) that provides the protection, effectively turning the circuit into an ordinary earthed circuit without the corresponding automatic disconnection arrangements.
Connecting multiple items of equipment to a single isolating transformer without the additional conditions (bonding between the enclosures, monitoring of the insulation condition) that apply in that case.
Never checking the insulation resistance of the separated circuit — a gradually deteriorating insulation can erode the protection without this being noticeable, until a second fault occurs.