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Overload And Overcurrent Relay

Shahid Hafeez

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Could any one tell me, the difference b/w Over current and overload relay.


An Over Load relay measure the current over time, also known as I^2t. It is not a linear current threshold, but rather a buildup of heat as measured by the energy (amps only) going into a load throughout a period of time. So a short duration but very high increase in current is not necessarily cause for tripping, but a low amount of extra current over a long period of time might be.

An Over current relay would look for just a specific current level and act upon that as a threshold response.


What the difference b/w Permissive under reach and Over reach Protection .

Not absolutely sure of the terminology (not used here in the US), but if I had to guess, I'd say that Permissive under reach would be a low limit threshold for an Current Sensing Relay, for example, a way to turn off a motor controller if a shaft or belt breaks and the motor stops drawing some minimum level of current. Over reach to me sounds like the opposite; a threshold that would be used to detect a jammed motor or some other condition that makes the running current exceed a set value.


what is the stub protection.


what is the SOTF protection.


No idea on either of these.

"He's not dead, he's just pinin' for the fjords!"
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