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Bus zone protection
Bus zone protection on HV switchgear
By john taylor on 4 May, 2007 - 12:51 pm


Can anyone explain the basics of bus zone protection on HV switchgear? Thanks.

By Black, Don on 6 May, 2007 - 12:34 pm

Go to the following web site

By Phil Corso, PE on 6 May, 2007 - 12:34 pm
1 out of 1 members thought this post was helpful...

John, the basic protection scheme used is measurement of differential-current. That is, if any of the incoming supply conductors carry fault-current but none of the outgoing feeders do, then the supplies are tripped. Simple,in principle, but difficult to implement!

Difficulty arises from the fact that there are many pit-falls. In fact, many times the decision's made to omit bus zone protection.

Let me know if additional information is required!

Regards, Phil Corso, PE (

John, I will try to explain what i know about busbar protection as i am not a protection engineer. On many installations busbar zone protection is not considered necessary or its use is limited to important substations.Apparently it is justified by the fact that a busbar fault is very unlikely and even if one should occur it would be cleared by another protection. Busbar protection must detect and clear faults within the zone and not trip for a fault at another location.

If all Current Transformers,all having the same ratio are mounted on every circuit connected to the busbar and are connected in parallel then a fault external to the busbar will result in a balance of all CT currents which will not initiate the high impedance relay,but if the fault is on the busbar then the protection will operate. Additionally to this where the design of the switchboard allows for a bus section switch which divides the switchgear into two sections.a fault on either side will trip that section only and leave the healthy section intact. It should be noted that in the bus section switch the CT`s are cross connected ie those on one side are connected to the other side and vice versa.This ensures that all the busbar,including the bus section switch itself will result in a complete loss of busbar. I hope this helps,let me know?


Everyone can read the T.Davies book on industrial power systems. Any ideas of your own?

By Subhro Roy on 7 May, 2007 - 10:23 pm


Bus zone protection is a unit protection (meaning a protection with a restricted zone, therefore does not need to be coordinated with other time based protections) specifically for MV/HV switchgear protection of Busbars and Circuit Breakers. Generally always Circuit Breakers are included in the zone of Busbar protection. They are of two types from point of view of principle of operation: High impedance and Low impedance types.

From the point of view of zones of protection they are classified into discriminating and check zone protections.

High impedance differential protection for the MV Switchgear Bus bar protection use the principle of comparison of currents by paralleling the secondaries of identical (both ratio and parameters) current transformers to a current operated relay and a stabilizing resistor in series, with a voltage limiting device across the combination. It is a very simple, reliable and fast operating scheme provided CTs and stabilizing resistors have been selected properly for stability against operation by through fault currents outside the zone of operation. For HV switchgears with more than one bus bars and switching isolators etc, the low impedance differential relay is used which is a costlier solution. However, it permits different CT ratios. These are numerical protection relays.

Please refer to the litrature of MCAG. MFAC and Micom P740 Relays from Alstom T&D web site for more details of High and low impedance Bus zone protection.

Dear John,

I will tell you something which I know related to your question.

In HV switchgear the zone of protection is segregated according to the position of the bus section or bus coupler. Mainly the selection of zone should be overlapping each other. Then only it will protect the system entirely from wherever the fault occurs.

For example, we have one bus coupler & 6 cable feeders on either side of the bus coupler. In this condition we need to provide 2 zones & 1 check zone. We have two trip relays on 2 different zones (zone 1 & 2). The CT connected to the corresponding zone will give initiation to the particular zone trip relay. Then the breaker will get trippped.