The number that matters is the actual earth-fault pickup current, not the multiplier by itself. If the instantaneous multiplier is referenced to the ground tap, 0.3 In × 30 = 9 In; changing to 0.6 In × 15 = 9 In preserves that instantaneous threshold but doubles the time-delayed earth-fault pickup. That trade usually sacrifices too much sensitivity, so retain the lower ground tap and use a coordinated time-delayed or IDMT earth-fault element unless the relay manual and protection study justify instantaneous operation.
Setting conflict and symptom quantities
The requested instantaneous multiplier is 30, while the relay permits no more than 20. With a 0.3 In ground tap, the desired threshold is 9 In, but the highest selectable threshold is 6 In if the multiplier is based on that tap.
| Quantity | Value or calculation | Where to confirm it |
|---|---|---|
| Existing ground tap | 0.3 In |
Earth-fault pickup setting |
| Requested instantaneous pickup | 0.3 In × 30 = 9 In |
Protection study |
| Relay multiplier limit | 20 |
Relay setting range or manual |
| Maximum pickup at the existing tap | 0.3 In × 20 = 6 In |
Derived after confirming the reference quantity |
| Proposed alternative | 0.6 In × 15 = 9 In |
Valid only when the multiplier follows the ground tap |
| Nominal relay input |
1 A or 5 A may apply |
Nameplate, order code, or manual |
A displayed multiplier outside the setting range is a design mismatch, not a relay malfunction. The decision is whether the desired 9 In threshold is necessary and coordinateable, not how to reproduce the same product by moving another setting.
Pickup-reference mechanism
First identify the multiplier reference. In some relays, instantaneous earth-fault pickup is a multiple of the selected earth-fault tap. In others, it is independent of the time-delayed overcurrent or earth-fault setting and instead references nominal relay current. The same displayed multiplier therefore can represent different secondary and primary amperes.
For a tap-referenced element, use:
Iinst,secondary = ground tap × instantaneous multiplier × In
For a nominal-current-referenced element, use:
Iinst,secondary = instantaneous setting × In
Convert either result through the installed CT ratio to obtain primary pickup. If a core-balance CT is used, apply its ratio and wiring arrangement rather than a phase-CT residual calculation. Read the relay manual’s setting definition before changing the tap; the front-panel numbers alone cannot identify the reference.
The Siemens 7SJ50 is an example cited for a design in which instantaneous pickup depends on the overcurrent or earth-fault setting. That behavior cannot be transferred automatically to another relay family or even inferred from a similar menu layout.
Earth-fault current and thermal mechanism
Earthing determines the available earth-fault current. A low-impedance return path can produce a high current, while an impedance-limited or otherwise restricted path may keep the current near the normal residual-current background. Obtain the minimum and maximum earth-fault currents at the relay location from the fault study, including relevant source and operating configurations.
This is current and heat, not logic. Fault current produces thermal stress proportional to current squared over time, while the protection system must still discriminate between the protected feeder and downstream faults. An instantaneous element removes intentional delay once pickup is crossed, so it can trip before a downstream device clears. That loss of grading is why instantaneous earth-fault elements frequently create nuisance operations or coordination failures.
The ground pickup also has to sit above standing residual current, phase-CT mismatch, transient spill current, and measurement error, yet below the minimum earth-fault current that the element must detect. A 0.3 pu pickup was already regarded as high for the described application, and 0.6 In increases the blind region further. A suggested time-delayed target was about 10% of the expected maximum current magnitude, but that value is a study input to test, not a universal rule.
Setting and coordination procedure
- Record the exact relay model, input rating, CT ratio, CT connection, and whether a core-balance CT supplies the earth-fault element.
- Read the setting definition for the instantaneous element. Classify it as ground-tap-referenced, nominal-current-referenced, or separately specified in amperes.
- Calculate the existing and proposed pickup values in relay secondary amperes. Convert them to primary amperes using the installed CT ratio.
- Obtain maximum and minimum earth-fault currents for each relevant earthing and source configuration. Compare minimum fault current with pickup and maximum current with interrupting and thermal limits.
- Plot the relay characteristic with downstream protective devices. Include the time-delayed or IDMT earth-fault curve, downstream clearing time, breaker clearing time, and applicable tolerances from the device data.
- If
6 Ininstantaneous pickup does not coordinate and9 Inis required, test whether a time delay or IDMT curve provides selectivity without losing earth-fault sensitivity. - Use
0.6 In × 15only when the manual confirms tap-referenced multiplication and the protection study accepts the resulting0.6 Intime-delayed pickup. - If the instantaneous setting references nominal current, leave the ground tap decision independent. Reassess the CT selection or relay range when the required primary threshold lies outside the available setting range.
Motor feeders and transformer feeders with delta-connected primaries were identified as applications where an instantaneous earth-fault element may warrant specific consideration. Application type alone is not permission to bypass grading; calculate the fault-current path and compare operating times.
Commissioning and verification
Verify the final values by secondary injection. Test immediately below pickup and above pickup, then record the measured operating current and time for both the instantaneous and time-delayed elements. The injected current must be interpreted through the same 1 A or 5 A nominal input and CT ratio used in the setting calculation.
Confirm that the relay indicates the intended earth-fault element rather than a phase-overcurrent element. Then perform a trip-path test through the assigned output, breaker trip circuit, and event record. Review the event or oscillography record for measured residual current, asserted element, trip output, and clearing sequence.
Repeat the coordination comparison using measured pickup and operating time rather than setting labels alone. Acceptance requires detection of the minimum protected-zone earth fault and slower operation than the downstream protection for faults beyond the intended zone, unless the protection design explicitly calls for simultaneous clearing.
Recurring setting pitfalls
| Pitfall | Consequence | Correction |
|---|---|---|
| Treating every multiplier as a multiple of the ground tap | Wrong primary pickup calculation | Read the relay’s multiplier reference |
| Raising the ground tap only to access another instantaneous value | Reduced sensitivity of the time-delayed element | Coordinate a delay or select suitable hardware |
| Ignoring the earthing method | Pickup may exceed available earth-fault current | Use minimum and maximum study currents |
| Checking pickup without downstream curves | Instantaneous overtrip and lost grading | Compare complete operating and clearing times |
| Using CT ratio as the only CT check | Residual error or saturation can drive unwanted operation | Review CT connection, performance, and test results |
FAQ
Can I use 0.6 In and 15 times instead of 0.3 In and 30 times?
Yes, but only when the instantaneous multiplier is referenced to the ground tap: both products equal 9 In. The change also doubles the time-delayed ground pickup from 0.3 In to 0.6 In, which can leave lower-current earth faults undetected.
Does a relay maximum of 20 times mean the CT must be changed?
Not automatically. Confirm whether the limit references the ground tap or nominal relay current, then convert the required pickup through the CT ratio; consider a CT or relay-range change only when the required primary pickup remains outside the available range.
Can I replace instantaneous earth-fault protection with an IDMT curve?
An IDMT or definite-time element is often the better choice when instantaneous operation defeats downstream grading. Plot the selected curve against minimum fault current and downstream clearing time, then verify it by secondary injection.
Does nuisance tripping mean the instantaneous pickup is too low?
It may indicate low pickup, CT residual error, transient spill current, incorrect wiring, or lost coordination. Stop changing settings when the relay reference quantity, earthing data, fault-current study, or CT performance is unknown. Escalate to the relay manufacturer’s official support channel with the exact model, settings file, CT details, injection results, and event records before returning the instantaneous element to service.