Table of Contents
What is CT operated overload relay?
Thermal overload relay is simple over current protection, it does not protect from single phasing or phase failure, short circuit, and earth fault. The operating principle of the relay is, when current flow through a conductor increases then I^2 X R loss also increases (Heat loss or ohmic loss).
Why do we use CT in the relay circuit?
CTs are needed because the circuit’s current is much higher than the relay can handle. CTs step down the current to a low level safe to connect to the relay. Like CTs, PTs are needed because the voltage in a circuit is much higher than the relay can handle, so they step it down to a much lower level for the relay.
What is CT connection?
CT (Current Transformer) Meters are installed on any connections with a load greater than 100 Amps. Conventional meters cannot be used to measure current for these larger connections as they would simply have to be too large to be practicable.
Why CT is used in panels?
The replica current is used as an input to a protective relay, which automatically isolates part of the power circuit, when a fault occurs. In doing so, the protection current transformer allows the unaffected areas of the circuit to continue in operation.
How do you set thermal overload settings?
Thermal Overload Relay setting = 100% x Full Load Current (Line).
- Thermal Overload Relay setting = 100% x Full Load Current (Line).
- Thermal Overload Relay setting = 100%x6 = 6 Amp.
How do you set a motor overload?
The overloads are determined using 125% of the FLA, 7A x 1.25 = 8.75A. The maximum allowable size for the overloads is 9.8A. The overloads can be sized at 140% of the FLA if the overloads trip at rated load or will not allow the motor to start, 7A x 1.4 = 9.8A.
How do I connect to a CT connection?
Key Points
- Install CTs on the phase conductor that corresponds to the voltage input phase.
- Install CTs with the arrow or label ‘This side toward source’ facing towards the breaker feeding the load.
- Connect the white and black CT leads to the corresponding CT input terminals with the white and black dots.
What is burden of CT?
The burden of the CT is the resistive load used to create the voltage on the output. It varies with turns ratio, desired output and current rating of the primary.
Why CT secondary is grounded?
If the secondary is not connected to a load, then it tries to maintain the counter magnetic field, and voltage levels may skyrocket. Levels can get so high, the air around the CT may start to break down and lose its insulating properties. This is why you should always make sure the secondary side of the CT is shorted.
How is CT ratio calculated?
Calculate the CT ratio. The CT ratio is the inverse of the voltage ratio. In this example, the voltage ratio is 1:5, so the CT ratio is 5:1. This means the current level is stepped down 5 times where, if the primary current is 200 amps, the CT output is 40 amps.
When to use a CT over load relay?
These types of relays are mainly used for low voltage and low load current applications (below the Full load current 200 Amps, < 110 KW). If your load current is more than 200 Amps better go for CT ( current transformer) operated relays which gives better performance than this type relays.
How does an overload relay protect the motor?
It senses overloading of motor and interrupts the power flow to the motor, thus protecting it from overheating and winding damages. Apart from overloads, it can also protect the motor from phase loss/ failures and phase imbalance. They are very commonly known as OLR. What is an overload?
What is trip class of overload relay in a circuit?
Overload relay is one such device that protects a motor from damages caused due to overloads and over-currents. It is used with contactors and can be found in motor control centers and motor starters. What is trip Class of overload relay? How to use an overload relay in a circuit?
Can a overload relay be set to high from Fla?
But if we setting to high from FLA,the overload relay cannot protect the motor if overload happen.It can cause the motor failure or winding burns.So we must calculated and get a right setting for the overload relay. How to setting Overload Relay protection? For adjustable protective devices, In corresponds to the value set.