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Topic Title: Earthing Switch -GIS High Voltage
Topic Summary: Fast Acting Earthing Switch
Created On: 24 December 2013 08:18 AM
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 24 December 2013 08:18 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

What are the differences between normal earthing switch and fast acting earthing switch?, it's possible to replace fast acting earthing switch with normal earthing switch or it's standard item?
 24 December 2013 10:33 AM
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Zuiko

Posts: 521
Joined: 14 September 2010

Not sure about the terminology, but by a fast switch do you mean a fault thrower?

There are two types of fault throwers

In HV distribution, these switches close one phase to earth in the event of a fault.

They are used on one end of a distribution circuit (usually 33 kV and above) instead of a circuit breaker and pilot wires.

What happens is that the circuit protection sees an in-zone fault - the end near the fault thrower and far from the feed - and closes the fault thrower. This closes one phase down to earth, which is seen at feeder end of the circuit (out of zone) by the protection as an earth fault which trips the feeder breaker at that end, then opens the isolators (with no load) at both ends. Depending on the scheme, the feeder breaker may then close.

It is done like this because the fault thrower is much cheaper to buy, install and maintain than a circuit breaker.



In LV systems the fault thrower will see a fault and instead of opening the contacts, which will draw an arc, the contacts close to earth. Arcs are non-ohmic, which is why they are so destructive (as the volts collapse the arc continues). By closing the contacts of the breaker to earth, the impedance of the fault is much lower. So instead of the fault current feeding a destructive arc, it is feeding a deliberate earth fault with relatively low current, which is seen by the upstream breaker which trips. The advantage here is that the fault currents are considerably lower, and the interruption times quicker.


The decision to use a fault thrower will depend on your budget, and how quickly you need to clear a fault.

cheers
W

Edited: 25 December 2013 at 05:28 PM by Zuiko
 24 December 2013 05:46 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

It could also be any switch that is closed to earth by charged springs rather than being manual-dependent.



Replacing a sprung-loaded switch with a manual dependent switch depends on the installation. At HV the preference is to earth via a circuit breaker (which are sprung loaded); and if a circuit breaker is not available, then via a switch (and if a switch is not available, then via a portable earth).

So I suppose if you do not have a CB, a sprung loaded switch is a safer option than a manual dependant one.

It's quite difficult to accidentally close an earth switch onto a live EHV circuit (what with the interlocking, and working to an approved schedule); but it is possible, and in these instances, you want the switch to make as soon as possible to trip the circuit. So a sprung loaded switch is safer; but really it should be in line with a CB. The switch should be closed when the circuit is not live*, then the CB closed to make the earth.




* Many Safety Rules define "dead" as "at or near earth potential". At HV and especially EHV, switching something off does not make it dead according to this definition, so you see the term "not live" for something that is no longer connected to a source of electrical energy, but is not at earth potential.

At HV and EHV, even when the circuit is not live, you can have quite significant capacitance on the line, and thousands of volts; so when you are closing to earth to discharge the circuit, you want to make it quick!

Edited: 25 December 2013 at 05:33 PM by Zuiko
 04 January 2014 02:46 PM
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smithi

Posts: 4
Joined: 16 February 2002

A fast acting earth switch is one which is mechanically charged (in modern switchgear usually by a spring). A charged spring via a mechanism drives the contacts of the switch together at a faster velocity than say manual operation. This allows the earth switch to be closed (not opened) onto a system which may have a fault on it. A fast acting earth switch is also used to discharge the cable or overhead line of the capacitive current which gets trapped when the circuit breakers are opened. A fast acting earth switch designed to IEC/EN 62271-102 is designed to close on to current which is 2.55 times the rms fault rating, a fault current of this magnitude is know as the making current. This type of earth switch can also be described as a 'fault make earth switch'. Other earth switches are manually operated or motor driven, so the speed the contacts touch are reliant on the operator or motor driven mechanism which is far to slow to close onto a fault. Hope this helps

-------------------------
smithi

Edited: 04 January 2014 at 02:55 PM by smithi
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