High Impedance Relay

From: http://www.crazyengineers.com/forum/electrical-electronics-engineering/28361-high-impedance-relay-vs-low-impedance-relay.html

Also see: http://www.howstuffworks.com/

"Low impedance relays are used to switch high currents(like motors) while high impedance relays are used to switch low currents

The reasoning is as follows.

Relays used to switch higher currents need thicker contacts (to minimise heat and power loss), so the thicker the contacts are the heavier they become. Now to attract the contact (to achieve switching operation) the magnetic attraction has to be more powerful (because of the additional weight), implying the relay's coil should generate a higher magnetic flux. To have a greater magnetic flux, the current through the coil should obviously be higher. To facilitate higher current in the coil the resistance (impedance) of the coil should be lower.

Low impedance relays are bigger and more expensive, so try to use higher impedance relays wherever possible."

you're so right..

Do you mean Differential Relays?

yes .especially REF.

Now, shall we get on to slugged relays. Slow operate or release. Contact area vs. current, coil windings, core and armature flux?

Please. My reference is correct for the average person who is not a relay designer!

§ The original question was about the difference between high impedance and low impedance relays.

§ No mention was made of "Differential".

§ However, differential relays have two current transformers feeding them, to detect unbalance between input and output grounded neutral. If the ground current rises, the relay operates and causes HT disconnection.

§ http://xnet.rrc.mb.ca/janaj/differential_protection.htm

§ High-impedance differential protection offers immunity against relay misoperation resulting from CT saturation. The SEL-587Z Relay uses internal 2000-ohm resistors to provide security against the effects of CT saturation for through faults. Internally mounted metal oxide varistors (MOV) are connected in parallel with the high-impedance element to clamp secondary peak voltage to less than 2 kV.

§ Low impedance relays have less protection against the possible HT voltages that may occur under fault conditions.

(From other sources)

High impedance relays are usually directly operated. Low impedance relays are often DC operated from a trickle-charged battery source which requires regular maintenance, often neglected.