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CT is used in the relay circuits for protection purposes. As the current and voltage ratings in a power system are very high, it is not possible to measure currents and voltages of that magnitude as they are way too large for any meter of reasonable size and cost. The CT’s are generally standardized at 1 or 5 amperes secondary current. These are very moderate ratings, thus a 5 A ammeter can be used to measure 1000 amperes with the help of a 1000/5 A ratio CT. For this reason the relay circuits are connected with instrument Transformers because the relay senses the input quantity before sending the trip signal to the circuit breaker, and these input quantities are measured by the use of instrument transformers.
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What is difference between isolation transformer and earth protection relay?
An isolation transformer is a mutual transformer having a voltage ratio of 1:1, so that the secondary circuit is electrically isolated from the primary circuit, yet subject to the same voltage.An earth protection relay is a category of protective relay that responds to an earth fault monitored by a current transformer (CT). In the event of an earth fault being detected by the CT, the relay with activate the trip circuit to a circuit breaker which will then disconnect the fault.
What is an electric circuit that controls another electric circuit through a relay?
Remote-Control Circuit. Any electrical circuit that controlsany other circuit through a relay or an equivalent device.
How an electrical relay works?
A relay will have a control circuit and a function circuitcould be the same feed line. One set of terminals (Hot and Load) can be activated at all times, but will not function until the control circuit is closed. The control circuit has a Hot and a Ground--when activated either by supply hot or ground by switch. The internal coil closes allowing the function circuit to operate. The purpose of this relay is to reduce the spike to the alternator. It takes less current (voltage) to close the coil on the relay than it would to operate the accessory that it controls
Why the secondary terminals of ct's are shorted when not in use?
The current in the secondary side of a CT (Current Transformer) is induced by the current flowing in the primary circuit. If the current continues to flow in the primary circuit and the CT is left "Open Circuit" then dangerous voltages can build up and cause damage to equipment when reconnected or personal injury by electric shock. Therefore, shorting the secondary terminals of the CT allows current to circulate taking flux with, thus reducing the building up of flux; the higher the flux the higher the emf (voltage) induced; E = Blv.
How do you built a mains fail alarm?
Use an intermediary relay that is specifically and only connected to a circuit from a two pole breaker in the distribution panel. Through the N.C. contact of the relay connect your DC alarm system. As long as the power is on the relay stays energized and the N.C. contacts stay open. If a power failure occurs the relay will drop out and close the N.C. contacts. This will close the circuit on the alarm system.
