Overcurrent Relays - Tap Setting vs. Tap Range 3

[EITengineering]

Hi Everybody,

I have a simple question regarding 50/51 overcurrent relays.

I understand that these relays have both a tap range and a tap setting within that range. Some of these ranges however overlap, i.e. Tap range 1.5-6.0 & 4.0-16. My question is if I have a tap setting of 5, does it matter which Tap range I choose? Are the characteristics of either range different given that my tap setting is 5 for both?

I searched the forums and I read one posting that the range is related to short circuit current. Is this range supposed to be a max withstand rating of the relay?


Thank you in advance for your help!

 

[dpc]

In general, the tap range is fixed based on the relay model, not a setting you can change. This is definitely the case for any electromechanical relays and for most digital relays that I can think of right now.

So you have to select the tap range the relay has and then select a tap setting (pickup) within that range.

 

[mgtrp]

In older electromechanical relays, it was normal to have a range similar to those that you've listed. Once the relay is purchased, you can't change this range. If your required setting falls in the overlap, then whether you pick the higher or lower range depends on where you think you might need future adjustment.

All that being said - in this day and age, I would expect you to be purchasing a numerical protection relay, with range so wide it's not an issue (actually, I don't recall every having to specify a range for a numerical relay, other than choosing between 1A and 5A CT secondaries).

 

[EITengineering]

Thanks for the answers gentlemen.

For the older style relays where several tap ranges are available, is there a difference in the characteristics (curves) of the same relay with different selected tap ranges? Or do they overlap over the values that the range is the same?

I ask because I'm conducting a short circuit study and the tap range for relays in the software do not match exactly those of the actual relay. Therefore, I'd like to know whether the pickup range will have an effect on the coordination curves of the relay.

I'm wondering if anyone can shed some light on that.

 

[rcw retired EE]

Taps change the pickup point, not the characteristic curve. There should not be any difference between a 4.0 tap on the low range and a 4.0 high range tap.

If there is a difference, it is within the same +/-% range as the normal variation in actual performance among a group of "identical" relays. Not something to worry about for your purposes. (If you are worried, you are measuring with a micrometer and cutting with an axe.)

 

[DTR2011]

The only thing I can think of that may be affected would be the burden the relay presents. You may want to verify this in the IB for the relays and see if the burden is different and if that has any effect on how you set it - this is all assuming an electromechanical relay