RCA =
45  °
X/R =
60  °

If fault current can flow in both directions through a relay location, it is necessary to add directionality to the overcurrent relays in order to obtain correct co-ordination. Typical systems which require such protection are parallel feeders (both plain and transformer) and ring main systems, each of which are relatively common in distribution networks.

In order to give directionality to an overcurrent relay, it is necessary to provide it with a suitable reference, or polarizing, signal.

The basic principle used in determining directionality is that, in a power system operating at unity power factor, phase current and phase-to-neutral voltage are in phase, and that phase current leads the phase-to-phase voltage between the other two conductors by 90° as can be seen below. Fault current in that conductor lags its phase-to-neutral voltage by the angle of system impedance. Therefore, the phase angle of fault current in Phase C will always lead the angle of the A-B voltage by an angle which can never exceed 90°. However, if the direction of the current in Phase C reverses, then its' angle will lag the A-B voltage by up to 90°.

It is therefore important to ensure the correct phasing of all current and voltage inputs to the relay, in line with the supplied application diagram. Under system fault conditions, the fault current vector will lag its nominal phase voltage by an angle dependent upon the system X/R ratio. It is therefore a requirement that the relay operates with maximum sensitivity for currents lying in this region. This is achieved by means of the relay characteristic angle (RCA) setting; this defines the angle by which the current applied to the relay must be displaced from the voltage applied to the relay to obtain maximum relay sensitivity.

Info

Directional Overcurrent (ANSI 67) Fault Analysis Tool

This is an interactive directional overcurrent (ANSI 67) reference and study tool. It visualizes how a directional overcurrent element uses voltage and current phasors, the relay characteristic angle, and the fault direction to decide forward or reverse and whether to trip. Adjust the inputs to see how the directional decision changes.

Who it is for

It is built for protection engineers and learners who want to build intuition for how a 67 directional element behaves.

Inputs and outputs

Typical use cases

What it does not do

It is an educational and reference visualizer, not a relay setting tool. It does not replace manufacturer documentation, your relay configuration software, or engineering judgment.

Privacy: This tool runs entirely in your browser. Nothing you enter is uploaded to a server.

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Frequently asked questions

What is ANSI 67 directional overcurrent?
ANSI device number 67 is a directional overcurrent element. It combines overcurrent pickup with a direction decision so the relay only operates for faults in a chosen direction, which is needed on parallel feeders and ring systems where fault current can flow either way.
How does a directional element decide direction?
It compares the operating current phasor against a polarizing voltage reference. The angle between them, relative to the relay characteristic angle, determines whether the fault is forward or reverse.
What is the relay characteristic angle?
The relay characteristic angle (RCA) is the angle by which the current must be displaced from the polarizing voltage to give maximum relay sensitivity. It is set to align with the expected fault current angle for the system.
Does this set my relay?
No. This is an educational and reference visualizer. It is not a relay setting tool and does not replace manufacturer documentation or your relay configuration software.
Is data uploaded?
No. The tool runs entirely in your browser and nothing is uploaded to a server.
What is it useful for?
It is useful for understanding 67 element behavior, checking directional logic concepts, and training on how phasors, the characteristic angle, and fault direction combine into a trip decision.