An old ASEA time-overcurrent relay whose nameplate reads "RI" (easily misread as "R1") is an induction-type relay with the RI characteristic. It was designed in 1918 and remained in service from about 1920 to 1985. The characteristic survives today as a mathematically defined curve in ABB and VAMP digital relays, where it is used for time grading against electromechanical relays. The modeling task in ETAP is therefore a curve-selection problem, not a hunt for a legacy data sheet.
RI curve equation and the quantities that set operating time
The RI characteristic is written in the source material as t(s) = k (0.339 - 0.236 x I>/I) with k = 0.05 to 1 and I> as the pickup setting. As typed, the expression has no division. The published RI form is the reciprocal:
t(s) = k / (0.339 - 0.236 * (I> / I))The physics decides which form is right. An inverse-time relay must operate faster as current rises. In the multiplied form, the term in parentheses grows as I rises, so t would increase with current, which is backwards. In the reciprocal form the denominator rises toward 0.339 as I/I> grows, and t falls toward a definite-time floor of k/0.339. Confirm the exact form against the SPAJ140 manual (equations on p. 19, curves on p. 24) or the VAMP 210 generator protection relay user's manual v005 (RI equation at PDF page 136).
| Quantity | Value / limit | Where to read it |
|---|---|---|
k (time multiplier) |
0.05 to 1 | Time dial on the relay face |
I> (pickup) |
Set point in amps, CT secondary or primary | Current tap or scale on the relay; settings sheet |
| High-current floor | k / 0.339 = 2.95 k (derived) | Limit of the equation as I/I> goes to infinity |
| Time at I = I> | k / 0.103 = 9.71 k (derived) | Equation limit; real relays need current above pickup to operate |
| Valid current range | Manufacturer-defined | Curve pages in the ABB or VAMP manual |
Three ways to represent the relay in ETAP
Three approaches apply. The comparison criteria that decide this case are curve fidelity, setup effort, and how well the result grades against remaining electromechanical relays.
| Approach | Fidelity | Effort | Comment |
|---|---|---|---|
| A. Library ABB relay with RI curve selected | Exact equation, maintained by the tool | Low | Many ABB models in ETAP include the RI curve |
| B. User-defined curve from the RI equation or points | Exact if points are computed from the equation | Medium | Use when no library device offers RI or when you need a custom label for the ASEA unit |
Use approach A. It reproduces the equation without hand-entered points. Use approach B only as an independent check on the plotted curve or when the library lacks a suitable device. Reject approach C for coordination studies, because grading margins against downstream devices depend on the time at high multiples of pickup.
Procedure for entering the ASEA RI relay
- Read the pickup setting
I>and the time dialkfrom the relay itself or its settings record. Note whether the tap is referenced to CT secondary or primary amps. - Add an overcurrent relay in ETAP and choose an ABB library device whose curve list includes RI.
- Select the RI curve type. Enter pickup as the equivalent primary or secondary current, matching the CT ratio in the model.
- Set the time multiplier to the dial value. If the library relay limits its multiplier range, compare it with the k range of 0.05 to 1.
- Plot the curve on the TCC and compare it with the computed points below.
Hand-check points from the equation
These times are derived from the reciprocal form above. Each is multiplied by the dial value k. The ratio is I/I> using the same current base for both.
| I / I> | t / k (s per unit k) |
|---|---|
| 1.0 | 9.71 |
| 1.5 | 5.50 |
| 2 | 4.53 |
| 5 | 3.43 |
| 10 | 3.17 |
| 20 | 3.06 |
A curve that flattens toward zero at high current indicates a wrong curve family or a definite-time setting.
Errors that shift the plotted curve
- Current base mismatch. Entering I> in primary amps while the fault current axis is secondary shifts the curve horizontally by the CT ratio.
- Nameplate misread. The marking is the letters R and I. A search for "R1" returns nothing useful, while a search for "ABB RI relay" returns the curve definitions.
- Electromechanical behavior outside the equation. Induction-disc relays have reset time and overtravel that the static equation does not represent. Add margin in coordination intervals for mixed EM and digital chains.
- Equation form. Entering the multiplied form as a user-defined curve produces times that rise with current. Recheck against the manual before saving points.
Confirming the model against the manual
- Open the SPAJ140 curve pages (p. 24) and read the RI curve at k = 1.
- Change k to 0.5 and confirm all times halve.
- Change I> and confirm the curve translates horizontally on a log current axis without changing shape.
Does the ASEA RI relay have a curve in ETAP?
The RI characteristic is included in many ABB relay models in ETAP, so select an ABB library device and choose RI as the curve type. If the library device lacks it, build a user-defined curve from the RI equation.
Yes. ABB and VAMP still implement the RI characteristic in current relays, and the equation is defined mathematically. The SPAJ140 manual gives it on p. 19, and the VAMP 210 user's manual v005 gives it at PDF page 136.
Can I use k values outside 0.05 to 1?
Keep k within 0.05 to 1 for the RI curve as documented. Read the dial range on the physical relay, since the settable range may be narrower than the equation range.
Does the RI curve reach zero time at high current?
No. From the equation, time approaches k/0.339, about 2.95 times k, as current rises. A plotted curve that continues to fall below this floor is not the RI characteristic.
Can I get the original settings or test data for a specific ASEA relay unit?
Ask ABB through its official support channels; ABB was formed in 1987 from ASEA and BBC and holds the protection relay history. Stop and escalate when the nameplate, dial range, or curve family cannot be matched to the manual curves, or when calibration test data is needed for a coordination study that carries safety or compliance consequences.