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Topics: How to select relay O/C + E/F Relay? on General Discussion
Start by
Rajesh Sain
09-06-2014 04:12 PM

How to select relay O/C + E/F Relay?

dear all, any one will suggest me, how to select relay O/C + E/F Relay and what is meaning of 50% 200% over current & e/f in C&R Panel, what is effect of system voltage??? kindly help me
09-06-2014 07:01 PM
Top #2
Gerbrand Ronsmans
09-06-2014 07:01 PM
Two possibilities:
a. The practical way: copy paste from a previous similar job and question if all the parameters are still the same and if some feedback came from the client. When changes apply -->question the impact, and make changes to the relay specification.
If no changes, order the same and enjoy the free afternoon.

b. The hard way:

Before you can start selecting the relays some preparation work has to be finished. Presumably the single line diagram is given and the switch-gear is determined.

You would start of with the protection concept, which differs based on the voltage level. In your case it seems like a medium voltage feeder.

Then you would need the results of the short circuit and load flow studies.

You have the ratings of the switch-gear, including current transformer data? Or will selecting the CT be part of the job?

You would also need the overall conceptual design. Will there be a auxiliary supply? Any restriction on dimension,...

You would need to select a vendor.

Then you would need to select a product range, usually based on the systems voltage level. Indirectly related to the budget.

Then you would need to specify the ordering code of the relay. Auxiliary voltage level, number inputs outputs required...

Did you mean C&P Control and Protection Panel? Maybe you need additional control functions? Communication protocol...

In general selecting the relay is a repetitive process. You make a first rough estimate on the requirements. Then you fill in some blanks get some input from the other designers/client. Refine the selection, update the engineering docs. Usually after two or three iteration you have the complete selection.
Typically all the data needed to select the relay is not yet available when you need it.

As far as the percentage, I have not heard of that. Unless in relation to generator protection. I assume maybe the setting is already given and it refers to 50% to 1.5In and 200% to 2.0In.
09-06-2014 09:27 PM
Top #3
Rajesh Sain
09-06-2014 09:27 PM
Dear Sir,

what is meaning of 50% 200% over current & e/f in C&R Panel, what is effect of system voltage???
09-06-2014 11:42 PM
Top #4
Martin Abreka
09-06-2014 11:42 PM
Hello, selection of OC and EF relay depends solely on: 1. The predetermined fault current on d system 2. The full load current or d primary current of d CT, 3. The sec. Fault Current of d CT( calculated using d system voltage and ct ratio) and 4. The PSM and the xteristics of d available relays. This applies to both OC & EF.
09-07-2014 02:04 AM
Top #5
Martin Abreka
09-07-2014 02:04 AM
50% and 200% means means coordinating ur relays @ 50 or 200% of the full load current. Dat is multiply d If by 0.5 or by 2. I think dis d little I know about question
09-07-2014 04:15 AM
Top #6
Rodney Hughes
09-07-2014 04:15 AM
% setting is not related to full load current specifically.
Relays have a nominal input current - usually either 1 A or 5 A
the setting range is expressed as a % of that nominal current.
Hence 50-200% of 1 A means 0.5 - 2 A whilst 50-200% of 5 A is 2.5 A - 10 A
The effective primary pick up setting is then given by the CT ratio
If it is say a 500/1 with a 1 A nominal current relay then setting the relay at 150% means that the relay is effectively set to 750 A primary.
The same is true if it was a 500/5 CT with a 5 A nominal current relay also set to 150%, i.e 750 A.
The full load current of course could be 700A or even 300 A or any other value - we just need to do a proper setting study which, as one small part, includes making sure the setting is above normal load current.

System voltage doesn't really come into it as such. The above example applies if the primary is 415 V, 11 kV or ... 500 kV. As far as OC relay setting is concerned, at least in this respect, we are only interested in knowing the full load current compared to the rated current of the CT. If full load is say 120% of the rated CT primary, then relay setting must be at least 120%, plus an appropriate margin, to make sure it won't operate for normal load.

One final comment - setting of OC relays is a very specific skill taking all sorts of factors into consideration to make sure that it won't operate for 'normal conditions but also that it will operate for required conditions. This includes the specification of the CT class itself and the pick up settings, curves and time multipliers. I would suggest you get a proper grading study done my specialists with experience in the process.
09-07-2014 07:04 AM
Top #7
Rajesh Sain
09-07-2014 07:04 AM
Dear Sir,

Thanks a lot for kind reply.

1. I think, 0.5 or 2. is decided by as per setting range of relay?

2. I am in practical condition where as:

Protection: Numerical, Non- Directional , 3 Overcurrent & 1 Earth fault Relay ,
CT Rating:- CTR - 10/5A
Application:- for Transformer 500KVA
Relay Module P111, Schneider,
Relay Setting in manual:

Phase : 0.20-40In, step 0.01In
Earth (ordering options):
0.01-2Ien (Model L: 0.01-1Ien), step 0.01Ien
0.05-12Ien (Model L: 0.1-8Ien), step 0.01Ien
Time delay: 0.0-200s (step 0.01s)

kindly suggest me what is input current for relay based on this available data.

09-07-2014 09:30 AM
Top #8
Rodney Hughes
09-07-2014 09:30 AM
1. Yes - the 0.5 - 2 is the range of available settings provided by the relay. Obviously each type of relay has certain step sizes for that setting eg you could set it at say 0.51, 0.52, ...

2. Some relays have the value of In as a setting itself - i.e. you could but a relay which you set for use with a 1 A CT or set it for use with a 5 A CT. Other relays you may need to buy the relay specifically for either 1 A or 5A.
So in the case of a relay being bought or set as In = 5 A
Then the Phase setting range 0.2 - 40 In means from 1 A to 200 A in steps of 0.05 A , i.e you could set it to 1.0, 1.05, 1.1, 1.15 ..... 199.9, 199.95, 200.0
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