MFAC Stabilizing Resistor: CT Stability Sets It, Not Metrosil

MFAC relay trouble usually appears as an unwanted bus differential operation during an external fault, failure to pick up during secondary injection, or uncertainty over which supplied resistor to install. Start with the relay nameplate and scheme drawing. The stabilizing resistor and the Metrosil solve different problems: the series resistor limits spill current during an external fault with CT saturation; the Metrosil limits voltage and absorbs energy during an internal fault.

Read the symptom before changing hardware

Take the first reading at the relay branch during the condition that exposes the problem. For a suspected stability issue, record the calculated or disturbance-derived secondary spill voltage for the maximum external through-fault. For a commissioning issue, record relay pickup voltage and current using the approved secondary-injection method.

Symptom	Likely cause and next check
Relay operates for an external bus fault	The saturated CT can drive more spill current than the relay setting permits. Check CT ratio, maximum external fault current, CT secondary resistance, lead-loop resistance, relay setting, and installed series resistance.
Relay pickup voltage is below the required stability voltage	The relay branch impedance is too low, the voltage setting is wrong, or the installed resistor does not match the scheme calculation. Calculate the minimum total branch resistance next.
Relay does not pick up at its documented setting	Look for excessive series resistance, incorrect wiring, open circuits, wrong taps, or a test method that bypasses part of the relay branch.
High secondary voltage appears during an internal fault	This is the Metrosil selection problem. Determine the internal-fault voltage and energy duty; changing only the stabilizing resistor does not establish the Metrosil rating.
A standard resistor size is listed but no selection rule is shown	Treat the listed sizes as available components, not automatic choices. Derive the required resistance from the stability voltage and relay operating current, then verify voltage, energy, and insulation duties.

Confirm the exact MFAC assembly

Do not calculate from the family name alone. Read the complete model designation, relay setting range, coil or element data, supplied resistor markings, and Metrosil identification. An MFAC 14/34 reference is usable only when the installed nameplate identifies that variant or the manufacturer confirms that its application data covers your assembly.

• Trace whether the stabilizing resistor is wired in series with the relay operating element.
• Trace whether the Metrosil is connected across the high-impedance relay circuit.
• Record every CT ratio used in the protected zone. Mixed or mismatched ratios invalidate a calculation that assumes identical secondary currents.
• Record the selected relay voltage or current setting rather than relying on a commissioning worksheet copied from another bus.
• Check whether the published relay setting already assumes a particular external resistor. Adding that resistor twice makes the relay less sensitive.

The GEC protective-relay application material identified for this problem discusses the stabilizing-resistor principle on pages 262-263. GEC modular protective-relay material, Section 3, pages 191-193, covers Metrosil characteristics and arrangement. Use the edition applicable to the installed relay because component options and setting conventions can differ.

Calculate the worst external-fault condition

The resistor calculation starts with the maximum external through-fault, not the maximum internal fault. During an external fault, healthy CTs drive secondary current while one CT can saturate. The saturated branch then behaves partly like its winding resistance and secondary-lead resistance, producing a voltage across the paralleled CT circuit. That voltage can force spill current through the differential relay even though the primary fault is outside the protected zone.

1. Obtain the maximum external primary fault current for the bus section and operating configuration being checked.
2. Refer that current to the CT secondary using the installed CT ratio: I_secondary = I_primary / CT ratio. Use consistent primary and secondary units.
3. Identify the saturated-CT current path from the actual schematic. Add the CT secondary winding resistance and the lead resistance present in that path.
4. Calculate the required stability voltage using the manufacturer-approved topology equation. A commonly encountered two-wire-loop form is , but use it only when the physical path contains both lead conductors represented by 2R_lead.
5. Repeat the calculation for every credible bus configuration. The resolving case is the one producing the highest required stability voltage.

Measure or calculate lead resistance for the actual conductor size, length, material, and operating temperature required by the project method. Guessing the cable resistance can move the result across the boundary between two available resistor sizes.

If the scheme drawing does not reveal whether its listed lead resistance is one-way or loop resistance, resolve that definition before multiplying it. Counting a loop value twice overstates the required voltage; treating a one-way value as the complete loop understates it.

Set total relay-branch resistance

At the calculated stability voltage, the spill current through the relay branch must remain below the selected relay operating current. Express that relationship as:

I_spill = V_stability / R_branch

For a simple series branch:

R_branch = R_relay + R_stabilizing + R_other-series

The minimum theoretical branch resistance is therefore:

R_branch,min = V_stability / I_setting

When the relay resistance and other series resistance are separately known:

R_stabilizing,min = (V_stability / I_setting) - R_relay - R_other-series

Use the relay manufacturer's definition of I_setting. It may not be interchangeable with a panel trip threshold or a current inferred from another relay variant. If the MFAC is specified by an operating-voltage setting, use the manufacturer's setting method and treat that voltage as the controlling relay quantity instead of independently reconstructing a coil-current setting.

Selecting the next available resistor above the calculated minimum can improve external-fault stability, but it also raises the voltage needed to produce relay operating current. Check the resulting sensitivity against the minimum internal bus-fault current. A resistor is not acceptable merely because it prevents operation for the maximum external fault.

Check resistor voltage and energy duty

Resistance is only the first selection criterion. Determine what the component must withstand while the protection operates and the fault is cleared.

• Calculate resistor current from the selected branch resistance and the applicable voltage.
• Calculate instantaneous resistor power as P = I²R or P = V²/R, applying each voltage to the correct component.
• Calculate pulse energy only when the clearing duration and current-versus-time behavior are defined. For a constant-current interval, E = I²Rt; do not use that shortcut for an undefined waveform.
• Check resistor voltage withstand, pulse-energy capability, insulation level, and installation clearances against the manufacturer's data.
• Check the complete relay circuit, not just the resistor body. Terminals, wiring, test switches, and insulation see the high-impedance scheme voltage.

A continuous wattage label alone does not establish short-duration fault capability. Likewise, a high pulse-energy rating does not establish adequate insulation or terminal voltage withstand.

Select the Metrosil separately

The Metrosil is a nonlinear voltage-limiting element. During an internal bus fault, the parallel CTs can develop a high secondary voltage across the high-impedance circuit. The Metrosil conducts increasingly as voltage rises, limiting the crest voltage and absorbing fault energy.

Base its selection on the maximum internal-fault energy to be dissipated. Obtain the nonlinear voltage-current characteristic, permissible energy, duty duration, recovery requirements, and connection arrangement from the applicable GEC data. The stabilizing-resistor calculation does not provide those values.

Keep the decision boundary clear:

• Use maximum external through-fault current and CT-circuit resistance to establish stability and the required series resistance.
• Use maximum internal-fault voltage, current, and clearing duty to establish Metrosil voltage and energy capability.
• Recalculate both duties after a CT ratio, CT type, lead length, fault level, relay setting, or clearing-time change.

Installing a larger Metrosil wastes time when the relay branch has insufficient stabilizing resistance. Installing a larger resistor wastes time when the actual problem is an incorrectly rated or disconnected Metrosil.

Install and verify the resolving branch

1. Confirm the complete MFAC model and obtain its applicable setting and component data.
2. Verify CT ratios, polarity, star-point arrangement, secondary continuity, and the boundary of the protected zone.
3. Calculate maximum external-fault secondary current for every relevant bus configuration.
4. Calculate the saturated-CT loop resistance and required stability voltage from the actual wiring path.
5. Calculate minimum total branch resistance, subtract documented existing series resistance, and choose an available stabilizing-resistor value that meets both stability and internal-fault sensitivity requirements.
6. Check the resistor's voltage, pulse-energy, insulation, and mounting duties. Select the Metrosil independently from the maximum internal-fault energy and voltage duty.
7. Wire the resistor in the documented series position and the Metrosil in its documented shunt position. Verify resistor markings and measured resistance before energization.
8. Perform the manufacturer's secondary-injection test. Record pickup voltage, pickup current, reset behavior, and operation of the connected trip logic.
9. Compare measured pickup with the selected setting and calculated stability requirement. Investigate wiring, resistance, settings, or test connections when the result falls outside the manufacturer's tolerance.
10. File the final calculation with CT data, lead resistance basis, fault levels, resistor identity, Metrosil identity, settings, and test results.

Do not prove stability by creating an uncontrolled primary fault. Use calculation, wiring checks, approved injection equipment, and the relay manufacturer's commissioning procedure.

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