Introduction
Electricity is one of the main component that is used to drive the machinery into production. Electricity is delivered by utility to an industry using over-head or under-ground conductors & cables. Electricity, even though is not visible is dangerous in terms of direct human contact, leaking through body of a device, arching between phases that are too close and so on. It is important that we handle electricity with caution.
There are mainly three types of faults that occur in any electrical system phase to phase fault or short circuit fault ; phase to neutral fault and phase to ground fault. Among the given faults the phase to phase is a severe type of fault, which when not attended has the potential to break-down the whole system to an extent that we need to replace most of the equipment, and in worst case harm any humans near the fault occurring area.
To avoid such faults or safeguard valuable resources (humans being at the top of that list) earthing rods, lightning arrestors, switchgears like MCCBs, ACBs, VCBs and so on are used. These protective devices in an industry can be compared to handrail for a staircase, importance of it is understood at rare but dangerous times. When a fault like phase to phase or phase to neutral short circuit occurs the usual current level jumps to several thousands of amperes (>16kA) which is far too dangerous for any appliance, let alone humans. To better understand it refer to fig. 1.
In this document, we will learn about the protective features provided by MCBs, MCCBs and ACBs to an electrical system.
1) Basic types of protections provided by switchgears.
Switchgears protect the electrical system against faults like,
Overload :
Over loading of system means the system is consuming current more than its rated current level, it may cause due to many different reasons. In such condition it is important to cut-off the supply to the faulty system or faulty appliance or else the system or an appliance will be partially or completely damaged depending upon the duration for which the system or appliance is subjected to over-load condition.
Short-circuit :
Short-circuit takes place between phase to phase or phase to neutral, phase to phase short-circuit is the most dangerous and devastating type of fault that can damage the system/appliance partially or completely. On short-circuit the current level exceeds 10,000 amperes, depending on location of fault and rating of transformer it can also exceed 55,000 amperes.
Short-circuit rating is also called as kA rating of a switchgear, as the current rating rises to some thousands of amperes
Earth fault :
Earth fault is recognized when when summation of current from three phases does not match current through neutral, in other words when current starts flowing through ground conductor. When ever any one of the phases is in contact with ground conductor or with the body, earth fault is occurred in the system.
2) Advanced types of protections
Introduction of micro-processors have added in on the list of protections provided by switchgears.
In a multi-tier system where, there are multiple distribution buses between source and load, it is important that the switchgear placed at higher tier (towards source) give time to switchgear placed at lower tier (towards load) to act upon faulty conditions. This need occurs because when the fault occurs, the faulty currents spreads from the point of fault to source in fraction of seconds. This may lead to multiple switchgear tripping, which in turn will result in shutting down the healthy part of the electrical system along with the faulty one which is undesirable. Here the micro-processor helps switchgear to delay it's action on faulty condition.
Delayed overload protection :
This type of protection helps switchgear give chance to switchgear placed in lower tier (towards load) to act upon over-load condition in a certain part of multi-tier electrical system which will result in successful shutting down of faulty part without hampering the working of healthy part of electrical system.
Delayed short-circuit :
This type of protection helps switchgear give chance to switchgear placed in lower tier (towards load) to act upon short-circuit condition in a certain part of multi-tier electrical system which will result in successful shutting down of faulty part without hampering the working of healthy part of electrical system.
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