The standard contact relay symbol consists of an actuator (the coil) and one or more switch contacts linked by a mechanical indicator or a shared alphanumeric device tag (such as K1 or CR1). In IEC 60617 schematics, the coil is represented by a rectangle, while North American NEMA/ANSI standards typically use a circle. Regardless of the standard, the direct answer to reading any relay schematic is this: first locate the coil tag, then find all contacts sharing that exact same tag across the drawing. They are mechanically linked, even if drawn inches apart on the page.
The Master Contact Relay Symbol Reference Table
Below is the definitive translation guide for the most common relay components across the three major drafting standards. Use this to cross-reference legacy panels with modern prints.
| Component | IEC 60617 (Global / EU) | NEMA / ANSI (North America) | Legacy BS 3939 (Old UK) |
|---|---|---|---|
| Relay Coil | Rectangle (often with diagonal line or 'K' label) | Circle (often with 'CR' or 'M' inside) | Circle with internal letter (e.g., 'R' for relay) |
| Normally Open (NO / Form A) | Two parallel lines with a gap, bridged by a diagonal line | Two parallel lines with a gap, bridged by a straight perpendicular line | Similar to NEMA, but often with a distinct hinge dot |
| Normally Closed (NC / Form B) | Two parallel lines bridged by a diagonal line with a cross-stroke | Two parallel lines bridged by a perpendicular line with a cross-stroke | Perpendicular bridge with cross-stroke and hinge dot |
| Changeover (COM / Form C) | A single moving line pivoting between two fixed contact lines | A single blade pivoting between two fixed contacts (often drawn as a 'Y' shape) | Pivoting blade with distinct hinge and two target lines |
| Time-Delay (On-Delay) | Contact symbol with an inward-pointing arrow (arrow to the line) | Contact symbol with an 'X' or specific timer box attached to the coil | Rarely standardized; usually a standard contact with a handwritten 'TD' note |
Rows People Get Wrong (and How to Fix Them)
Even experienced bench technicians misread specific rows in relay schematics. Here are the most common errors and how to avoid them on the jobsite.
1. Confusing Time-Delay Arrows
In IEC schematics, a time-delay contact is indicated by an arrow. The direction of the arrow dictates the behavior. An arrow pointing toward the contact line means on-delay (the contact waits to change state after the coil is energized). An arrow pointing away from the contact line means off-delay (the contact changes state immediately on energization, but waits to revert when the coil drops out). Mixing these up will cause sequential motor starters to fire out of order.
2. Form C vs. Two Separate Contacts
A Form C (SPDT - Single Pole Double Throw) contact relay symbol shows a single moving element (the Common terminal) switching between an NO and an NC terminal. Beginners often confuse this with two separate Form A (SPST) contacts drawn close together. The practical difference is critical: a Form C contact is inherently break-before-make (or make-before-break, depending on the specific relay model like the Schneider Zelio series). Two separate contacts operated by the same coil have no guaranteed mechanical sequencing. Never use a Form C symbol to represent two isolated circuits.
3. The Dashed Mechanical Link
In older NEMA drawings, a dashed line connects the coil to its associated contacts. Novices frequently trace this dashed line with a multimeter, assuming it is a control wire. It is not. It is purely a graphical representation of the physical armature inside the relay casing. If you see a dashed line, do not look for a wire; look for the physical relay housing.
Regional Standards and Faded Panel Protocols
Which standard applies to your bench? If you are working in Europe, Asia, or on modern globally-sourced OEM equipment, you will predominantly see IEC 60617 symbols. If you are in North America, dealing with legacy MCCs (Motor Control Centers), or reading prints governed by NFPA 70 (NEC) and NEMA standards, you will see circles and distinct switch blades. In the UK, panels built before the late 1990s may still feature BS 3939 symbols.
Safe Interpretation When Markings are Faded
Control panels in harsh environments (like wastewater treatment or lumber mills) often suffer from faded schematic labels and oxidized relay tags. If the 'K1' ink is completely gone from both the coil and the contacts, do not guess based on wire colors—control wire colors are not universally standardized for relay coils.
The Bench Protocol:
- De-energize the circuit and verify dead.
- Identify the physical relay (e.g., an Omron MY2N 8-pin octal).
- Use the physical pinout as your source of truth. On a standard 8-pin octal, pins 2 and 7 are always the coil. Pins 1, 3, and 4 form one contact set (1=COM, 3=NO, 4=NC), while 8, 6, and 5 form the second (8=COM, 6=NO, 5=NC).
- Use your multimeter in continuity mode. Place one probe on the suspected Common terminal and the other on the NO terminal. Manually press the relay's test button (the small plastic tab on the top of most industrial relays). If the meter beeps, you have positively identified the NO contact, regardless of what the faded schematic says.
Frequently Asked Questions
What does the dashed line mean in a contact relay symbol?
The dashed line in a contact relay symbol represents a mechanical linkage, not an electrical wire. It indicates that the movement of the relay's internal armature (driven by the coil) physically pushes or pulls the switch contacts. In modern IEC drafting, this dashed line is often omitted in favor of identical alphanumeric tags (like 'K1.1', 'K1.2') to keep schematics clean, but you will still see it frequently in North American NEMA prints and legacy diagrams.
How do I identify a contact relay symbol if the coil marking is faded?
If the schematic tag is unreadable, abandon the drawing and trace the physical hardware. Lock out the power, remove the relay from its socket if possible, and read the manufacturer part number printed on the casing (e.g., "Omron MY2N-D2 DC24"). Look up the manufacturer's datasheet for the exact pinout. Alternatively, use a multimeter's continuity function to map the coil pins (which will show a resistance typically between 50 and 2000 ohms, depending on the voltage rating) and the switch contacts (which will read 0.0 ohms when manually actuated and OL when at rest).
What is the difference between Form A, Form B, and Form C relay contacts?
These terms describe the physical switch configuration inside the relay. Form A is a Normally Open (NO) contact; it is open when the coil is de-energized and closes when energized. Form B is a Normally Closed (NC) contact; it is closed at rest and opens when the coil pulls in. Form C is a Changeover (SPDT) contact, which combines both: it has a Common (COM) terminal that rests against the NC terminal at rest, and switches over to the NO terminal when the coil is energized. Form C is the most common configuration in industrial control relays because it offers maximum wiring flexibility.
Why do some schematics show the contact relay symbol coil and contacts in completely different parts of the drawing?
This is a feature of 'ladder logic' or 'line diagram' drafting, standard in North American industrial controls. The goal of a ladder diagram is to show the logical flow of current from Line (L1) to Neutral (L2) in straight vertical and horizontal lines, making it easy to troubleshoot with a voltmeter. Because a single relay coil might control three different motors in three different parts of the machine, drawing the contacts physically next to the coil would create a chaotic web of crossing wires. By separating them and linking them purely by a shared tag name (e.g., 'CR1'), the schematic remains readable and logical, even though the physical components are housed in the same plastic shell on the DIN rail.
