Knowledge Base Article #167 - Pickup values on Microprocessor relays that FAIL with minimal pickup settings
DateMar 31, 2008
ByENOSERV
Filed UnderRTS General
ClassGeneral
 
Issue Addressed
When testing an operate value where the relay pickup setting may be 0.5 amps or less, you can get a fail situation when basing the Min and Max range on percentages only.

Let's assume you are testing a relay at 0.5 amp pickup, and the relay operates at 0.43 amps. When calculating tolerances for a pickup of 0.5 at a manufacturer allowance of +/- 3%, this would require the relay to pickup at 0.485-0.515. This would cause the relay to FAIL. If you stretched the tolerances to +/- 5%, the range would be 0.475-0.525 which is also a FAIL. For the sake of arguement you set the tolerances at +/- 10%, so the range would be 0.450-0.550, which is still a FAIL. What do you do? Is the relay bad?
 
Causes of Issue
The relay may not be bad or out of the manufacturers tolerance levels. Generally the relay manufacturer will give two tolerance values, one in %, the other in +/-.

In the case of an SEL-751A relay, the accuracy of the Instantaneous elements are listed at +/- 5% of setting +/- 0.02*INOM secondary. In this case they are first calculating the % error allowed and then extending that value by 0.02 amps for a 1 amp sec relay, 0.1 amps for a 5 amp sec relay.

Considering the same situation, the +/- 5% of a 0.5 amp pickup is 0.465-0.525, then taking into account the +/- 0.1 amps, the range is extended to 0.365-0.625, which now allows a pickup of 0.42 amps to be a pass situation
 
Solution to this Issue
With the release of RTS version 4.3.6.0, we have added in the ability to handle this situation, but have not added the options to the interface so that it is visible. In the mean time, you can add the flags MIN_ABSTOL and MAX_ABSTOL in Advanced Options of a FasTest Routine and define the values as needed. This is an example of code that was added to the recently completed SEL-751A.FST routine.

'SEL ALLOWS % OR +/- TOLERANCE. +/- EVALUATION IS REQUIRED WHEN PICKUP < 1.0 AMPS
MIN_ABSTOL=-0.1
MAX_ABSTOL= 0.1

In the current state, RTS will first evaluate by % entries. If a FAIL has occurred, the progam will look for the new flags to determine if the element willl PASS using the absolute toerances.

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