- ELR -96
ABM - INSTRUMENT
ABM - INSTRUMENT
An Earth Leakage Relay (ELR) with a Core Balance Current Transformer (CBCT), such as the widely availableÂ
ELR 96
 model, is a crucial safety device used to detect and isolate small current leaks to the ground, preventing electric shocks and fire hazards. The "96" in the name typically refers to its standard panel-mounted dimension of 96 x 96 mm.Â
Key Features and Specifications (common forÂ
ELR 96
 models)
- Function:Â Continuously monitors for leakage currents and, if they exceed a preset level, trips an associated circuit breaker (MCB, MCCB, or ACB) via a shunt or under-voltage release coil.
- Dimensions:Â Standard 96 x 96 mm panel-mount size (depth varies).
- Input Signal Range:Â Typically adjustable, ranging from as low as 30 mA up to 30 A AC through an external CBCT.
- Trip Time Delay:Â User-selectable/adjustable time delay, often from instantaneous to several seconds (e.g., 0 to 10 seconds).
- Display:Â Many models feature a digital LED display to show the real-time leakage current value and fault conditions.
- Output:Â One or more relay contacts (e.g., 1 C/O or DPDT) with a specified contact load (e.g., 5A, 230V AC) for tripping and/or alarm purposes.
- Auxiliary Power Supply:Â Wide range of AC or AC/DC auxiliary voltage options (e.g., 100-250V AC, 24-48V AC/DC).
- Additional Features:
- Test/Reset Function:Â Built-in push buttons for manual/remote testing and resetting the device.
- CBCT Open Detection:Â Monitors the connection to the CBCT for reliability.
- Fail-Safe Option:Â Ensures the line is protected even if the auxiliary power to the relay is lost.
- Fault Memory:Â Stores the last fault value or condition for later review.Â
Working Principle
The ELR with CBCT operates on the core balance principle (also known as zero-sequence current detection):Â
- Current Monitoring:Â All live (phase) and neutral conductors of the circuit being protected are passed through the central aperture of the ring-type CBCT.
- Imbalance Detection:Â Under normal, healthy conditions, the vector sum of the currents in all conductors is zero. The CBCT produces no signal.
- Leakage Identification:Â If an earth fault or leakage occurs (e.g., a person touches a live wire, or insulation breaks down), some current flows to earth, bypassing the CBCT. This creates an imbalance, and the vector sum is no longer zero.
- Signal Transmission and Tripping:Â The CBCT detects this residual current imbalance and sends a proportional signal to the ELR. When this signal exceeds the user-set sensitivity limit (trip current), the ELR activates its output relay contacts, which in turn de-energize the shunt trip or under-voltage release coil of the main circuit breaker, thereby isolating the power supply
In the context of electrical engineering and specifically for the ELR 96 with CBCT, the term "capacity work" most likely refers to the rated switching capacity (or breaking capacity) of the relay's output contacts or the associated circuit breaker it controls. It can also broadly refer to the electrical load the overall protection system is designed to handle.Â
Here is a breakdown of the relevant concepts:
1. Rated Switching Capacity of the Relay Contacts
This is a critical specification for any relay and dictates the maximum electrical load (voltage and current) that the relay's internal contacts can safely switch (open and close) without damage like excessive arcing or welding.Â
- Expression:Â It is typically expressed in Volt-Amperes (VA) for AC circuits and Watts (W) for DC circuits.
- Significance for the ELR 96:Â The ELR 96 itself is a control device. Its output contacts are designed to handle the relatively small current and voltage required to energize the trip coil (shunt release or undervoltage release) of a main circuit breaker. A common rating might be around 5A at 230V AC. The relay is not designed to directly interrupt the large main power supply current of the protected circuit itself.Â
2. Breaking Capacity of the Associated Circuit Breaker
The ELR 96 is always used in conjunction with a main circuit breaker (e.g., MCB, MCCB, ACB) which is the device that actually interrupts the high-power main supply when a fault is detected.Â
- Breaking Capacity (Interrupting Rating):Â This is the maximum fault current that the circuit breaker is certified to interrupt safely at a specific voltage. This is the actual "capacity work" of interrupting the main circuit in the event of an earth leakage fault. This value is much higher than the relay's contact rating and is a primary consideration when designing a protection system.
- Making Capacity:Â This is the maximum peak current the breaker can withstand and successfully close against (usually higher than the breaking capacity).Â
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