Earth Leakage Relay Symbol

Definition: The Earth Leakage Relay symbol represents a protective relay that monitors residual ground-fault current in an electrical installation and issues a trip signal when leakage current exceeds a set threshold, as defined in IEC 60755 and IEC 60947-2, with the designator K (relay) or RCD on schematic diagrams.

Also known as: ground fault relay, RCD relay, residual current relay, earth fault relay, leakage current relay, ground leakage relay.

What the Earth Leakage Relay symbol means

The Earth Leakage Relay symbol denotes a sensing and switching device that continuously measures the vector sum of all line and neutral currents through a toroidal current transformer; any imbalance (ground fault or leakage current) above the set trip level — typically 30 mA for personnel protection or 100–500 mA for equipment protection — causes the relay to energise a trip coil or open a circuit breaker.

On industrial and electrical schematics, this symbol appears in the protective-relay circuit alongside a current transformer (CT) on the supply cable and a trip contact wired in series with the circuit-breaker shunt-trip or contactor coil. The relay plays a critical safety role in preventing electric shock and electrical fires caused by insulation breakdown.

How to identify the Earth Leakage Relay symbol

The earth leakage relay symbol is drawn as a rectangle (the standard relay block convention) with the label 'ELR', 'RCD', or a delta/residual-current symbol (sometimes ΔI) inside. Three pins are shown: Line input (L, left side), a +24 V or auxiliary supply (VCC, top), and a Trip output (right side) connected to a downstream protective device. The symbol may also show a toroidal CT around the monitored conductors, with a dashed line linking the CT secondary to the relay block.

Function and usage boundary

The earth leakage relay measures residual current — the algebraic sum of instantaneous current values in the phase and neutral conductors — using a toroidal core current transformer. Under normal conditions the sum is zero; a leakage path to earth creates an imbalance that the relay detects. When the imbalance exceeds the set threshold (Idn), the relay output activates a trip signal on the Trip pin, opening the upstream breaker or contactor within the specified operating time (typically <300 ms for 30 mA personnel-protection types per IEC 61008).

Standards: IEC vs ANSI

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Logical terminals

Where the Earth Leakage Relay symbol is used

• Industrial motor-control panels where a 100–300 mA earth leakage relay protects equipment and wiring against insulation degradation
• Commercial and residential distribution boards with 30 mA RCDs on socket-outlet circuits for personnel shock protection per IEC 60364-4-41
• Medical electrical installations requiring 10 mA or 30 mA trip levels per IEC 60601-1 to protect patients in body-contact zones
• PV solar installations where DC leakage-current sensing relays detect insulation faults in the array before they cause fire
• Fire-detection-level protection: 300–500 mA earth leakage relays on final circuits to detect slow insulation degradation that can ignite cable insulation
• IT-system (isolated earth) monitoring using insulation monitoring devices (IMDs) that trigger a leakage alarm before the first fault causes a trip

Key facts

• The Earth Leakage Relay monitors residual current (the vector sum of phase and neutral currents) using a toroidal CT; any imbalance indicates current flowing to earth via an unintended leakage path.
• IEC 60755 classifies residual current devices by AC/DC waveform sensitivity: Type AC (AC only), Type A (AC + pulsating DC), and Type B (AC + smooth DC) — motor drives and EV chargers require Type B per IEC 61009.
• Personnel-protection trip level is 30 mA per IEC 61008 / IEC 60364-4-41; at 30 mA, ventricular fibrillation risk is low but the current can still cause shock, so the relay must trip within 300 ms at Idn.
• ANSI/IEEE device function number 64 identifies a ground-detector relay on one-line diagrams; IEC schematics use the residual-current block symbol with ΔI notation.
• The relay has three key connection points: Line input (L) from the monitored circuit, auxiliary supply (+24 V / VCC), and a Trip output that activates a circuit-breaker shunt-trip or contactor de-energisation coil.
• Earth leakage relays differ from MCBs and fuses: they respond to ground-fault current (as low as 10 mA) rather than overcurrent; a device can pass a 20 A load safely but still trip on 30 mA of leakage.
• IEC 60947-2 Annex M requires periodic test-button verification; the test button injects a simulated imbalance to confirm relay and trip-coil operation without creating a real fault.

Frequently asked questions

What does the earth leakage relay symbol mean in a circuit diagram?

The earth leakage relay symbol marks a protective device that monitors the residual (leakage) current in a circuit and opens the circuit when leakage exceeds a set threshold (e.g. 30 mA). It is drawn as a relay rectangle labelled 'ELR' or 'RCD' with Line, Trip, and auxiliary supply pins.

What is the difference between an earth leakage relay and an RCD?

An RCD (residual current device) is a self-contained switch/relay unit that both detects leakage current and interrupts the circuit. An earth leakage relay is only the sensing/tripping relay; it detects the fault and sends a trip signal to a separate circuit breaker or contactor. Both are governed by IEC 60755 and IEC 60947-2.

What trip current level does an earth leakage relay use for personnel protection?

For personnel shock protection the standard trip threshold is 30 mA per IEC 61008 and IEC 60364-4-41. At this level the relay must operate within 300 ms at the rated Idn current. Higher thresholds (100–500 mA) are used for equipment and fire protection where personnel contact is not the primary concern.

What is the ANSI device function number for an earth leakage relay?

ANSI/IEEE C37.2 assigns device function number 64 to ground-detector relays. On one-line diagrams the relay block is labelled '64' inside a rectangle. IEC schematics instead use a residual-current block symbol with the ΔI designation.

What does the Trip pin on the earth leakage relay symbol do?

The Trip pin carries the relay output signal that activates a shunt-trip coil, contactor de-energisation coil, or alarm when leakage current exceeds the set threshold. In the symbol, the Trip pin (x=50, y=20) connects to the downstream protective device to open the faulty circuit.

Why does an EV charger require a Type B earth leakage relay?

EV chargers can produce smooth DC leakage currents from their power electronics. Type A residual current devices only detect AC and pulsating DC faults and will not respond to smooth DC leakage. IEC 61009 and IEC 60364-7-722 therefore require Type B devices (sensitive to AC, pulsating DC, and smooth DC) for EV charging installations.

How does the earth leakage relay symbol differ from a standard overcurrent relay symbol?

An overcurrent relay symbol (ANSI function 51) responds to excessive load current flowing in the normal circuit path and uses a current transformer in series with the line. An earth leakage relay symbol includes a toroidal CT enclosing all conductors and responds only to residual (imbalance) current that bypasses the load via an earth path.

Related symbols

• RCD / RCCB symbol
• Relay Coil symbol
• Relay Module symbol
• Relay NC Contact symbol
• Relay NO Contact symbol
• Relay SPDT symbol

Physical breadboard, PCB, and panel mapping

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