Chapter 2 – Time Overcurrent (51) Element Testing
B) Pickup Test Procedure
Use the following steps to perform a pickup test if the setting is less than 10 secondary Amps:
¾ Determine how you will monitor pickup and set the relay accordingly, if required. (Pickup indication by LED, output contact, front panel display, etc…see previous packages of The Relay Testing Handbook for details)
¾ Set the fault current 5% higher than the pickup setting. For example, 8.40 Amps for an element with an 8.00 Amp setpoint. Make sure pickup indication operates.
¾ Slowly lower the current until the pickup indication is off. Slowly raise current until pickup indication is fully on. Chattering contacts or LEDs are not considered pickup.
Record pickup values on test sheet. The following figure displays the pickup procedure.
STEADY-STATE PICK-UP TEST 4 A
8 A
ELEMENT PICK-UP
PICKUP SETTING 12 A
Figure 12: Pickup Test Graph
C) Pickup Test Procedure if Pickup is Greater Than 10 Amps
Use the following steps to determine pickup if the setting is greater than 10 secondary Amps:
¾ Check the maximum per-phase output of the test set, and use the appropriate connection shown in Figures 7-11. For example, if the 50-element pickup is 35 A and your test set’s maximum output is 25amps per phase; use “High Current Connections #1.” If the pickup setting is greater than 50amps, use “High Current Connections #2.” If the pickup is higher than 75 A (3x25A), you will have to use another test set or temporarily lower the setting.
Remember, setting changes are a last resort.
¾ Determine how you will monitor pickup and set the relay accordingly, if required. (Pickup indication by LED, output contact, front panel display, etc…see previous packages of The Relay Testing Handbook for details)
¾ Set the fault current 5% higher than the pickup setting. For example, set the fault current at 42.0 Amps for an element with a 40.0 Amp setpoint. Apply current for a moment, and make sure the pickup indication operates. If pickup does not operate, check connections and settings and run the test again until the pickup indication operates.
¾ Set the fault current 5% lower than the pickup setting. Apply current for a moment and watch to make sure the pickup indication does not operate. Increase and momentarily apply current in equal steps until pickup is indicated. If large steps were used, reduce the amount of current per step around the pickup setting. See the following figure for a graph of this pickup method.
JOGGING PICK-UP TEST 40 A
ELEMENT PICK-UP
PICKUP SETTING 20 A
60 A
Figure 13: Pickup Test Graph - Jogging
D) Test Procedure to Avoid Setting Changes and Interference
It can be easier and more practical to test 50-elements without changing settings or disabling elements. The 50-element time delay setting is usually very small. The 50-element should trip before the time overcurrent (51) at the 50-element pickup level. The following procedure allows 50-element pickup testing without changing settings.
¾ Determine which output the 50-element trips and connect timing input to the relay output.
¾ Check the maximum per-phase output of the test set and use the appropriate connection from figures 7-11 in this chapter. For example, if the 50-element pickup is 35 A and your test set can only output 25amps per phase; use “High Current Connections #1.” If the pickup setting is greater than 50amps, use “High Current Connections #2.” If the pickup is higher than 75 A (3x25A), you will have to use another test set or temporarily lower the setting. Remember, setting changes are a last resort.
¾ Set the fault current 5% higher than the pickup setting. For example, set the fault current at 42.0 Amps for an element with a 40.0 Amp setpoint. Set your test set to stop when the timing input operates and to record the time delay from test start to stop. Apply test current and ensure the relay output stopped the test and note the test time. Compare the test time to the 50-element time delay setting to ensure timing is correct. Review relay targets to ensure the correct element operated.
¾ Set the fault current 5% lower than the pickup setting. Apply test current and watch for timing input operation. If the relay does not operate after the 50-element time delay, stop the test manually. If the timing input operates, ensure the time delay is longer than the 50-element and review targets to ensure the 50-50-element did not operate. Increase and apply current in increasing steps until the 50-element time delay is observed. If large steps were used, lower the current below the pickup setting and use smaller steps to achieve better resolution.
4. Timing Tests
There is often a time delay applied to the 50-element protection even though the 50-element is defined as instantaneous overcurrent protection. Timing tests should always be performed even if time delay is not assigned.
50-element timing tests are performed by applying 110 % of pickup current (or any value above pickup) to the relay and measuring the time between the start of the test and relay operation. The start command could be an external trigger, a preset time, or a push button on the relay set. The stop command should be an actual output contact from the relay because that is what would happen under real-life conditions.
1 2 3 4 5 6 7 0
TIME IN CYCLES 2A
4A 6A 8A 8.8A
PICK UP
Figure 14: 50-Element Timing Test
When the 50-element time delay is zero or very small (less than 2 seconds), the actual measured time delay can be longer than expected. There is an inherent delay before the relay can detect a fault plus an additional delay between fault detection and output relay operation. These delays are very small (less than 5 cycles) and are insignificant with time delays greater than 2 seconds.
The first delay exists because the relay is constantly analyzing the input data to determine if it is valid and this analysis takes a fraction of a cycle. The relay cannot determine the magnitude of the input signal until it has enough of the waveform to perform an analyze and determine the rms or peak current or voltage. The relay is also a computer and computers can only perform one task at a time. If a fault occurs just after the relay processes the line of code that detects that particular fault, the relay has to run through the entire program one more time before the fault is detected.
All of these delays usually require a fair portion of a cycle to complete. The “Operate Time” and
“Timer Accuracy” specifications in the following figure detail this time delay.
PHASE / NEUTRAL / GROUND IOC Current: Phasor Only
Pickup Level: 0.000 pu to 30.000pu in steps of .001 pu Dropout Level: 97% to 98% of Pickup
Level Accuracy: +/- 0.5% of reading or +/- 1% of rated (Whichever is greater) from 0.1 to 2.0 x CT ration +/- 1.5% of reading > 2.0 x CT rating < 2%
Overreach: < 2%
Pickup Delay: 0.00 to 600.00 in steps of 0.01 s Reset Delay: 0.00 to 600.00 in steps of 0.01 s Operate Time: < 20 ms @ 3 x Pickup @ 60Hz
Timing Accuracy: Operate @ 1.5 x Pickup +/- 3% or +/- 4ms (whichever is greater)
Figure 15: GE D-60 Relay Overcurrent Technical Specifications
The second time delay occurs after the relay has detected the fault and issues the command to operate the output relays. There is another fraction of a cycle delay to evaluate what output contacts should operate and then the actual contact operation can add up to an additional cycle depending on relay manufacturer, model, etc. “Operate Time” in the following figure represents this delay for the specified relay.
FORM-C AND CRITICAL FAILURE RELAY OUTPUTS Make and Carry for 0.2 sec: 10 A
Carry Continuous: 6 A
Break @ L/R of 40ms: 0.1 ADC max Operate Time: < 8 ms Contact material: Silver Alloy
Figure 16: GE D-60 Relay Output Contact Technical Specifications
Your test set can also add a small time delay to the test result as shown by the “Accuracy”
specification of the following figure:
MANTA 1710 TIME MEASUREMENT SPECIFICATIONS Auto ranging Scale: 0 – 99999 sec
Auto ranging Scale: 0 – 99999 cycles Best Resolution: 0.1 ms / 0.1 cycles
Two wire pulse timing mode
Accuracy: 0 – 9.9999 sec scale: +/-0.5ms +/- 1LS digit all other scales: +/- 0.005% +/- 1 digit
Figure 17: Manta Test Systems M-1710 Technical Specifications
What does all this mean? With a time delay of zero, the time test result for a GE D-60 relay, using a Manta M-1710 test set, could be as much as 32.6 ms or 1.956 cycles as shown in the following figure:
Minimum Time Test Result Relay Operate Time: < 20 ms
Relay Timing Accuracy: +/- 4ms Relay Operate Time: < 8 ms
Test Set : +/-0.5ms
+/- 1LS digit (0.1 ms) 32.6 ms or 1.956 cycles
Figure 18: 50-Element Minimum Pickup
A) Timing Test Procedure
¾ Determine which output the 50-element trips and connect timing input to the output.
¾ Check the maximum per-phase output of the test set and use the appropriate connection from figures 7-11. For example, if the 50-element pickup is 35 A and your test set can only output 25amps per phase; use “High Current Connections #1.” If the pickup setting is greater than 50amps, use “High Current Connections #2.” If the pickup is higher than 75 A (3x25A), you will have to use another test set or temporarily lower the setting.
Remember, setting changes are a last resort.
¾ Set the fault current 10% higher than the pickup setting. For example, set the fault current at 44.0 Amps for an element with a 40.0 Amp setpoint. Set your test set to stop when the timing input operates and to record the time delay from test start to stop.
¾ Apply test current and ensure timing input operates and note the time on your test sheet.
Compare the test time to the 50-element timing to ensure timing is correct.
¾ Review relay targets to ensure the correct element operated.
¾ Repeat for other two phases.
5. Residual Neutral Instantaneous Overcurrent Protection
Residual neutral overcurrent protection is typically set well below phase overcurrent values. In these cases, follow the previous steps but apply current in one phase at a time. It is good practice to perform pickup tests on A-phase and timing tests on B and C-phases to make sure the relay uses all three phases to calculate residual current.
If the phase overcurrent settings interfere with residual testing or the pickup results are not as accurate as they should be, connect the relay and test set as shown earlier in Figure 7, but apply all three phase currents simultaneously at the same phase angle. The magnitude of each phase should be one-third of the test current. Some relay models need currents through all three phases to accurately calculate residual current.
6. Tips and Tricks to Overcome Common Obstacles
The following tips or tricks may help you overcome the most common obstacles.
¾ Before you start, apply current at a lower value and review the relay’s measured values to make sure your test set is actually producing an output and your connections are correct.
¾ If the element does not operate, watch the metering during the test if possible.
¾ Check to make sure your settings are correct.
¾ Make sure you are connected to the correct output.
¾ Check the output connections by pulsing the output and watching the relay input.
¾ Some relay test-set-inputs are polarity sensitive. If the connections look good, try reversing the leads.
¾ Have any of your test leads fallen off?
¾ If you are paralleling more than one relay output, do all channels have the same phase angle?
¾ Check for settings like “Any Two Phases” (Any two phases must be above the pickup to operate) or “All Three Phases” (All three phases must be higher than the pickup to operate) or
“Any Phase” (Any phase above pickup operates element).
¾ If you need more than one phase to operate the 50-element but your test set only has enough VA for one phase, put two or more phases in series as shown below:
A Phase Amps
Input ElementOutput - TimerInput
+ DC Supply
Figure 19: 50-Element Alternate Relay Connection
¾ Check for blocking inputs.
¾ Does the relay need breaker status or other input to operate?
Sometimes neutral or residual ground protection is applied and this protection is inevitably set lower than the phase elements. These elements trip first before the phase element operates and can be a nuisance at best. The following solutions can help overcome this obstacle:
¾ Perform tests using three-phase, balanced inputs as shown in the “Neutral or Residual Ground Bypass Connection.” Residual current will be zero.
¾ Perform tests with three phase inputs with two phases slightly below the pickup and slowly raise one phase at a time until pickup is indicated.
¾ Apply a phase-phase fault by applying equal current to any two phases with the current applied 180º from each other as per Figure 11. For example, a 25 Amp pickup could be tested by applying 25 Amps @ 0º in Phase A-N and 25 Amps @ 180º into Phase B-N.
Chapter 2
Time Overcurrent (51) Element Testing
1. Application
The 51-element is the most common protective element applied in electrical systems. It uses an inverse curved characteristic and will operate more quickly as the fault magnitude increases.
There are many different styles of curves in use and each style mimics a different damage characteristic. The most common curve characteristics used in North America are usually described as ANSI or U.S. Curves. Some relays allow you to select European curves usually described as IEC curves. All of these curves are mathematic models of electro-mechanical relays to allow coordination between different generations of relays. General Electric used special curves for their IAC electromechanical relay line and some relays also have these curves available. Custom curves could also be available to create specific protection curves unique to a an individual piece of equipment (like motors), but this feature is seldom used. Examples of the different styles of curves with identical settings are shown in Figure 20. Notice that the x-axis values represent a multiple of the element’s pickup setting. This is typical so that all curves can be plotted without site-specific values. Most manufacturers display their curves in multiples of pickup or its equivalents - “percent of pickup” or “I/Ipkp”
Time Coordination Curve
0.10 1.00 10.00 100.00
1 10 100
Multiple of Pickup Current
Time in seconds
Extremely Inverse
Normally Inverse
Very Inverse
Moderately Inverse
Figure 20: 51-Element North American Curves Time Coordination Curve
0.10 1.00 10.00 100.00 1,000.00
1 10 100
Multiple of Pickup Current
Time in seconds
IEC Standard Inverse IEC Very Inverse IEC Extremely Inverse IEC Long-Time Inverse IEC Short-Time Inverse
Figure 21: 51-Element IEC European Curves
After the appropriate curve style is chosen, 51-elements typically have two primary settings, pickup and timing. The pickup setting changes the starting point of the curve. As the pickup setting increases, the curve moves from left to right as shown in the following TCC of an ANSI Extremely Inverse (EI) curve with different pickup values.
Time Coordination Curve
1.00 10.00 100.00
1 10 100
Secondary Amps
Time in seconds
1 Amp Pickup 2 Amp Pick up 3 Amp Pickup 4 Amp Pickup 5 Amp Pickup
Figure 22: ANSI Extremely Inverse with Different Pickup Settings
The curve moves vertically as the time dial setting is increased as shown in the following figure of an ANSI Extremely Inverse curve with different time dials.
Time Coordination Curve
1.00 10.00 100.00 1,000.00
1 10 100
Secondary Amps
Time in seconds
Time Dial 1 Time Dial 2 Time Dial 3 Time Dial 4 Time Dial 5
Figure 23: ANSI Extremely Inverse with Different Timing Settings
2. Settings
Typical settings for 51-elements are described below:
A) Enable Setting
Many relays allow the user to enable or disable settings. Make sure that the element is ON/Enabled or the relay may prevent you from entering settings. If the element is not used, the setting should be disabled or OFF to prevent confusion.
B) Pickup
This setting determines when the relay will start timing. Different relay models use different methods to set the actual pickup and the most common methods are:
¾ Secondary Amps – the simplest unit. Pickup Amps = setting
¾ Per Unit (P.U.) – This setting could be a multiple of the nominal current as defined or calculated if the relay has setpoints for nominal current, Watts, or VA. It could also be a multiple of the nominal CT secondary.
Pickup = Setting x Nominal Amps, OR
Pickup = Setting x Watts / (nominal voltage x √3 x power factor), OR Pickup = Setting x VA / (nominal voltage x √3), OR
Pickup = Settings x CT Secondary (typically 5 Amps)
¾ Primary Amps – There must be a setting for CT ratio if this setting style exists. Check the CT ratio from the drawings and make sure that the drawing matches the settings.
Pickup = Setting / CT Ratio, OR
Pickup = Setting * CT secondary / CT primary
C) Curve
This setting chooses which curve will be used for timing. Be very careful to select the correct curve as there can be subtle differences between curve descriptions. Compare the curve selection to the coordination study to ensure the correct curve is selected
D) Time Dial/Multiplier
This setting simulates the time dial setting on an electro-mechanical relay and sets the time delay between pickup and operation. ANSI curves usually have a time delay between 1 and 10. IEC time dial setting are typically between 0 and 1.
E) Reset
Electro-mechanical 51-element relay timing was controlled using a mechanical disc that would rotate if the current was higher than the pickup setting. If the current dropped below the pickup value, the disc would slowly rotate back to the reset position. The disc speed in the trip and reset directions are directly related to the amount of current flowing through the relay.
Some digital relays simulate this reset delay using a linear curve that is directly proportional to the current to closely match the electro-mechanical relays. Other relays have a preset time delay or user defined reset delay that should be set to allow any electro-mechanical discs to reset for proper coordinate between devices.
3. Pickup Testing
Time overcurrent testing is theoretically simple. Apply current into the appropriate input and increase until you observe the pickup indication. However, the actual application can be complicated and requires some imagination because 51-element testing can overlap with neutral over-current protection and timing tests can interfere with 50-element testing. Look in the tips and tricks heading of this section for ways to avoid interference from other elements.
Write down all settings related to the 51-element and calculate what the pickup current should be using the formulas described in the previous section.
Now that you have determined the pickup and time delay settings, convert the current to primary values using the following formulas:
¾ Primary Current = Secondary pickup current * CT ratio, OR
¾ Primary Current = Secondary Pickup current * CT Primary / CT Secondary.
It is extremely unlikely that you will find a microprocessor relay out of calibration. We perform these tests to check relay operation and verify that the engineer has correctly interpreted the settings. Check the primary values and time delays against the coordination study and make sure they match. Make sure the supplied TCC curves are at the correct voltage levels as discussed in previous packages of The Relay Testing Handbook. If you do not have the coordination study, quickly check that the upstream relay 51-element pickup and timing settings are higher and the downstream relay 51-elements settings are lower. Use voltage conversions discussed in previous packages of The Relay Testing Handbook if necessary.
A) Test Set Connections
Because of possible high currents involved with timing tests, you may need to try some of the alternative test set connections shown in Figures 24-28.
Because of possible high currents involved with timing tests, you may need to try some of the alternative test set connections shown in Figures 24-28.