The telephone influence factor (TIF) weights each harmonic by how much it interferes with a telephone line. The I·T product multiplies the rms current by the TIF. Annex B of IEEE 519-2022 gives three reference categories for distribution networks up to 34.5 kV with shared poles; they are not limits.
Short answer
The telephone influence factor (TIF) measures how much a distorted signal interferes with a telephone line: it weights each harmonic by the effect it has on the ear and the telephone set. The I·T product is the rms current (I, in amperes) times the TIF (T). IEEE 519-2022 treats them in its informative Annex B, with three reference categories for distribution networks up to 34.5 kV. Those categories are not limits.
What the TIF is
Clause 3.1 defines the TIF for a voltage or current wave of a supply circuit: the ratio of the root of the sum of the squares of the weighted rms values of all sinusoidal components (including the fundamental and the harmonics) to the unweighted rms value of the whole wave.
Annex B explains how the weighting is done. The weighting combines two effects:
- C-message weighting, which represents the relative interference effect of the different frequencies in the voice band, including the response of the telephone set and the ear.
- A capacitor, which gives a weighting directly proportional to frequency to represent the assumed coupling function.
The TIF is a dimensionless quantity that indicates the shape of the wave and not its amplitude. Two currents with the same shape and different amplitude have the same TIF; that is why it is combined with amplitude in the I·T product.
The weights for each frequency (Table B.1) are based on the C-message weighting of the 1960s. The standard allows linear interpolation between the table values. For more information it refers to IEEE Std 776-2018.
The I·T product and the kV·T product
Telephone interference is often expressed as the product of current and TIF:
| Indicator | Definition | Units |
|---|---|---|
| I·T product | Rms current (A) multiplied by its TIF | Amperes (the TIF has no unit) |
| V·T or kV·T product | Rms voltage multiplied by the TIF; in the kV·T the voltage is in kV | Volts or kilovolts |
Clause 3.1 defines the I·T product as the inductive influence expressed as the product of the rms current and its TIF, and the V·T product equivalently with voltage.
The categories of Table B.2
Annex B.1 gives reference guidelines for the I·T product in distribution systems up to 34.5 kV, where joint use of facilities, in particular poles and structures, between power and telephone or communications companies is most likely.
| Category | Description | I·T |
|---|---|---|
| I | Levels very unlikely to cause interference | Up to 10,000 |
| II | Levels that could cause interference | From 10,000 up to 25,000 |
| III | Levels likely to cause interference | Over 25,000 |
Two notes accompany the table:
- Type of exposure. The values correspond to circuits with exposure between overhead systems, both power and telephone. Inside an industrial plant or commercial building, the exposure between power cables and twisted-pair telephone lines is extremely low and normally there is no interference. Using optical fiber cables removes practically all concern.
- Earth return. In some areas that use earth return in telephone or power circuits, the 10,000 value of category I can be as low as 1,500.
A reading example
A 13.2 kV overhead feeder shares poles with a copper telephone line. The feeder’s rms current is measured and its TIF is calculated with the Table B.1 weights: its I·T product is the product of both. With a result of 8,000 (a made-up figure for illustration), it falls in category I; with 18,000, in category II; with 30,000, in category III. With earth return, category I can start lower, down to 1,500.
The figure by itself is not a compliance result: it indicates interference potential. If the value is high and there are noise complaints on the line, it is worth measuring on the shared-exposure stretch and studying the case with the telephone company.
When it matters and when it does not
The Table B.2 categories refer to exposure between overhead power and telephone systems, which is where the standard places them. Inside industrial plants and buildings the exposure is very low, and with optical fiber the concern disappears in practice. The TIF becomes relevant when a distribution feeder shares poles or structures with copper telephone circuits.
Also remember that the standard does not cover radio-frequency interference; it only offers guidance for wire-line telephone systems (clause 1.1).
Relation to the harmonic limits
The I·T product neither replaces nor complements the limits of Tables 1 to 4 of IEEE 519. The distortion limits are evaluated at the PCC with the statistical criteria of clause 5; the TIF is a comparison tool for cases of exposure with telephone lines. For the limits, see IEEE 519 Harmonic Limits: Key Concepts, PCC and TDD and THD: What It Is, How It Is Calculated, IEEE 519 Limits.
The TIF is not a limit of the standard. The harmonic limits apply at the PCC and are in IEEE 519-2022 Voltage Harmonic Limits at the PCC and Harmonic Current Limits and TDD by Isc/IL; the scope of the standard, including the guidance for wire-line telephony, is explained in IEEE 519 Scope: Who Is Responsible for Harmonics.
We help with your case
At Tiza Group we carry out power quality studies, in Colombia, or elsewhere on request: if your feeder shares poles with communications networks and there are signs of interference, we measure the current harmonics, calculate the I·T product with the IEEE 519-2022 weights, and assess the measures with you.
Related guides
- IEEE 519 Harmonic Limits: Key Concepts, PCC and TDD
- IEEE 519 Scope: Who Is Responsible for Harmonics
- IEEE 519-2022 Voltage Harmonic Limits at the PCC
- Harmonic Current Limits and TDD by Isc/IL
- Power Quality in Industry: Problems and Causes
References
- IEEE Std 519-2022, IEEE Standard for Harmonic Control in Electric Power Systems: clause 3.1 (TIF, I·T product, V·T product); informative Annex B (weighting, Table B.1, B.1 guidelines for the I·T product, Table B.2 and its notes); clause 1.1 (interference in wire-line telephone systems).
- IEEE Std 776-2018, IEEE Recommended Practice for Inductive Coordination of Electric Supply and Communication Lines.
Frequently asked questions
Is the I·T product a limit in IEEE 519?
No. Annex B is informative, and the standard clarifies that its guidelines should not be considered recommended limits, because of the wide variability of systems and equipment. The three categories serve to illustrate and compare interference potential between areas of the same system.
Does the TIF have units?
No. It is a dimensionless quantity that depends on the waveform and not on its amplitude. The I·T product does have units of current (amperes times the dimensionless factor), and the kV·T product uses the voltage in kV.
Should I worry about TIF inside my industrial plant?
The note to Table B.2 states that inside an industrial plant or commercial building, the exposure between power cables and twisted-pair telephone lines is extremely low and normally there is no interference. Using optical fiber removes practically all concern.