Category Archive for: «Protection»

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Things About MCBs That Every Low Voltage Electrician Should Know

MCB Operation and characteristics An MCB is a thermo-magnetic device, meaning that it has two methods of circuit interruption. A thermal mechanism, usually a bi-metallic strip, provides protection against moderate overcurrent. The heating action of the current causes the bi-metallic strip to curve and break circuit contact. This method is complemented by a solenoid designed to respond to […]

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Selective Coordination with Circuit Breakers

Introduction to Selective Coordination Selective coordination is often referred to simply as coordination. Coordination is defined in NEC® 240.2 as: “The proper localization of a fault condition to restrict outages to the equipment affected, accomplished by the choice of selective fault-protective devices.” The figure below shows the difference between a system without selective coordination and […]

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The Philosophy Of Breaker Failure Protection

High speed protection scheme The separation of equipment into zones will only work successfully if the outputs of the protections can be guaranteed to do their job. This makes logical sense. For the bus protection in Figure 1 for example, the breakers X and Y must trip on a fault, otherwise the adjacent zone providing […]

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Current and voltage sensors as an alternative to traditional CTs and VTs

Sensors as alternative As an alternative for traditional primary current and voltage measurement techniques, the use of sensor technique is gaining field. This technique is typically applied to current and voltage measurement in medium-voltage metal-enclosed indoor switchgears. There are many undeniable advantages with sensors when compared to the traditional solutions: Non-saturable High degree of accuracy […]

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Understanding The Ratios Of Magnetic HV Instrument Current Transformers

Primary & secondary windings The primary winding is a part of the network, carrying the actual load current. The transformer’s secondary circuits usually consist of several units, each of them having its own magnetic core and winding. The transformer has a rated current transforming ratio, like for example 200/1 A, stating the rated primary current […]

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Basics of Built-in Motor Protection for Beginners

Why is motor protection necessary? In order to avoid unexpected breakdowns, costly repairs and subsequent losses due to motor downtime, it is important that the motor is fitted with some sort of protective device. This article will deal with built-in motor protection with thermal overload protection to avoid damage and breakdown of motor. The built-in […]

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3 Overcurrent Protective Device Ratings You Must Properly Select

Ampere, voltage and interrupting rating In order for an overcurrent protective device to operate properly, the overcurrent protective device ratings must be properly selected. These ratings include: Voltage rating Ampere rating and Interrupting rating – Minimum Interrupting Rating. If the interrupting rating is not properly selected, a serious hazard for equipment and personnel will exist. Current […]

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A real life analogy of the jurisdiction of the protection relay

To Protect the Transmission Line Under Fault Formally, a relay is a logical element which processes the inputs (mostly voltages and currents) from the system/apparatus and issues a trip decision if a fault within the protection relay‘s jurisdiction is detected. A conceptual diagram of relay is shown in Figure 1 below. Let’s see an example […]

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Voltage Imbalance Protection of Synchronous Generator (ANSI Code 60)

Synchronous Generator Protection In an apparatus protection perspective, generators constitute a special class of power network equipment because faults are very rare, but can be highly destructive and therefore very costly when they occur. If for most utilities, generation integrity must be preserved by avoiding erroneous tripping, removing a generator in case of a serious […]

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Miniature Circuit Breakers (MCBs) For Beginners

Dual functions of a switch and a fuse This technical article focuses on the widely used miniature circuit breakers rated for 240 V or less. These are smaller and lower-rated versions of industrial-grade molded-case circuit breakers rated for 600 V or less. These miniature breakers, called air circuit breakers, are based on thermal, magnetic, or […]

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