Electrical switching symbols — reading the IEC 60617 legend

The reading a single-line diagram guide and the reading a distribution board schematic guide cover what type of document you are looking at and how it is structured. This article covers what is needed to actually read such a diagram: the meaning of the individual symbols themselves, standardised under IEC 60617 (historically known in the Netherlands as NEN 5152).

The core symbols

Symbol (function)	Meaning
Make contact (NO)	A contact that is closed (makes a connection) when the operating element is activated.
Break contact (NC)	A contact that is open (breaks a connection) when the operating element is activated.
Changeover contact	A combination of a make and a break contact, linked to one operating element.
Coil (relay/contactor)	A rectangle with terminals, often carrying a letter code (for example K1) that identifies the associated contact elsewhere on the diagram.
Disconnector	A diagonal line without additional marking — may only be operated de-energized or unloaded.
Load-break switch (disconnector)	A diagonal line with an additional mark indicating the device may be switched under load.
Circuit breaker	A diagonal line with a cross mark — the cross mark indicates the device trips automatically on overcurrent or short circuit, unlike a disconnector or load-break switch.
Fuse	A rectangle (sometimes with a diagonal line through it).
Motor	A circle with the letter M.
Transformer	Two (partly overlapping) circles.
Earth	A series of horizontal, progressively shorter dashes.

The difference that actually matters: the cross mark

The distinction between a disconnector and a circuit breaker looks small at first glance — both are drawn as a diagonal line in the main current path — but is functionally fundamental:

A disconnector has no automatic tripping function: it only switches when a person operates it, and, as a rule, may only be opened or closed while de-energized.
A circuit breaker (the device with the cross mark) trips automatically on overcurrent or short circuit, without anyone needing to intervene.

Mistaking these two functions for the same device — for example assuming that a disconnector drawn on the diagram also automatically protects an installation against a short circuit — connects directly to the functional confusion described in the §537 classification guide: not every switching symbol performs the same protective function, even though the drawing on the diagram may look very similar.

Regional and historical variation

Not every diagram you come across uses exactly the same symbol set. Older drawings (produced before the full transition to IEC 60617) may still contain the older, national NEN 5152 symbols, which differ in some details from the current IEC symbols. Small visual differences also exist between different CAD/schematic drawing packages for how the same function is represented.

Note: when in doubt, always check the legend printed on the specific diagram (or that of the drawing package it was made in), rather than blindly trusting a symbol you recognise from another project.

Practical relevance

When reviewing a single-line diagram or distribution board schematic before starting work, it must first be established which type of switch (disconnector, load-break switch or circuit breaker) each drawn symbol actually represents — this information is directly relevant to whether a device may be operated under load and whether it automatically protects against a fault, both relevant to the LOTO five-steps guide for safely de-energizing part of an installation.

Common mistakes

Confusing a disconnector and a load-break switch based on a superficial glance at the symbol — the difference (whether or not it may be switched under load) is safety-critical and not always obvious at first sight from the drawing itself.
Assuming every diagonal line in the main current path automatically protects against a short circuit — only the device with the cross mark (circuit breaker) does; a disconnector or load-break switch does not.
Interpreting a diagram from another project or an older drawing without checking the legend — symbols can differ in detail by period, country or drawing package.