IEC Relay Test Standards for 220V Auxiliary Supply: What You Need to Know

A protection relay doesn’t work without its auxiliary supply. The CT and VT measurements feed the analog inputs, but the processor, the display, the communications, and the trip contacts all run off the auxiliary voltage. In most of the world, that auxiliary voltage is 220V — either 220V DC from a substation battery or 220V AC from a UPS-backed control bus.

If the auxiliary supply drops out, brows out, gets ripple from a failing battery charger, or spikes from switching transients, the relay can misoperate, restart during a fault, or simply stop working. This is why the IEC 60255 series defines specific tests for auxiliary supply behavior, separate from the protection function tests.

This article covers the IEC standards that govern testing protection relays operating on 220V auxiliary supply — what each standard actually tests, the correct voltage tolerances, and how to commission and maintain the auxiliary supply circuit.

What “220V Auxiliary Supply” Means in Practice

“220V auxiliary supply” is a loose term that covers several different real-world scenarios. Before testing, you need to know which one applies to your installation.

220V DC (station battery)

The most common configuration in utility substations and heavy industrial plants. A battery bank (typically lead-acid or Ni-Cd) provides a nominal 220V DC, float-charged by a battery charger connected to the station AC supply. Actual battery voltage varies from about 198V (at end-of-discharge) to 242V (at float charge).

This is the configuration addressed by IEC 61000-4-29 (voltage dips, short interruptions, and variations on DC power ports) and IEC 61000-4-17 (AC ripple on DC power ports). Both standards are referenced by IEC 60255-26 for protection relay EMC testing.

220V AC (UPS or control bus)

Common in industrial LV installations, commercial buildings, and some utility control rooms. A UPS provides 220V (or 230V in many European installations) AC on the control bus, backed by batteries.

This is the configuration addressed by IEC 61000-4-11 (voltage dips, short interruptions, and variations on AC power ports).

Dual-rated aux supplies

Many modern numerical relays accept a wide range of auxiliary voltages — for example, “60–250V DC / 110–240V AC” on a single input. These relays are designed to work on 24V, 48V, 110V, 125V, 220V, or 240V without reconfiguration. Testing requirements don’t change with the rating, but you need to know where in the range your actual supply sits.

Where 220V sits in the IEC-preferred values

IEC 60255-1 lists preferred rated auxiliary voltages: 24V, 48V, 60V, 110V, 125V, 220V, 250V. A relay rated for 220V nominal operates on the 220V preferred value. Some manufacturers use 230V or 240V as effectively equivalent ratings.

Applicable IEC Standards (And Which One Does What)

Multiple IEC standards apply to protection relay auxiliary supply testing. Understanding exactly which standard covers what saves time — and prevents the common mistake of attributing tests to the wrong standard.

Standard	What It Covers
IEC 60255-1	Common requirements — defines rated auxiliary voltage, preferred values, and basic requirements
IEC 60255-26	EMC requirements for protection relays — specifies which EMC tests apply and at what levels
IEC 60255-27	Product safety — covers dielectric tests and touch voltages related to the aux supply input
IEC 61000-4-11	AC voltage dips, short interruptions, and variations (AC power ports only, 50/60 Hz)
IEC 61000-4-17	AC ripple superimposed on DC power input ports
IEC 61000-4-29	DC voltage dips, short interruptions, and variations (DC power ports only)

The key distinction most people get wrong

AC ripple on DC and voltage dips on DC are two different standards:

• IEC 61000-4-17 = AC ripple (the 100/120 Hz content from a battery charger’s rectifier)
• IEC 61000-4-29 = voltage dips and interruptions (sudden changes in the DC voltage level)

They test different phenomena. A failing battery charger produces more ripple (IEC 61000-4-17 territory). A main AC feed fault causing the charger to drop out momentarily produces a voltage dip (IEC 61000-4-29 territory). Both matter, but they’re separate tests.

For a 220V DC aux supply on a protection relay, IEC 61000-4-29 and IEC 61000-4-17 are the primary references.

Rated Auxiliary Voltage and Tolerance (IEC 60255-1)

What the standard specifies

IEC 60255-1 defines the rated auxiliary energizing voltage (U_aux) as the value declared by the manufacturer for which the relay is designed to operate correctly within specified tolerances.

The standard doesn’t prescribe a single universal tolerance. Instead, the manufacturer declares the tolerance range, which is typically:

• Minimum: 80% of rated (176V for a 220V-rated relay)
• Maximum: 120% of rated (264V for a 220V-rated relay)

So for a relay rated at 220V, correct operation should be maintained over the range of 176V to 264V. This is often called the “80% to 120%” or “0.8 × U_n to 1.2 × U_n” specification.

Wide-range auxiliary supplies

Modern numerical relays often specify much wider ranges — for example, 88V to 300V DC or 110V to 250V AC. These wide-range supplies are designed to accept any IEC-preferred voltage within the range without reconfiguration. For a 220V application, a relay with a rated range of 88–300V DC provides significant margin.

What happens outside the rated range

• Below minimum: The relay may reset, lose settings, or produce indeterminate behavior. Modern relays should enter a defined “power fail” state and refuse to operate. Older electromechanical relays might chatter or trip spuriously.
• Above maximum: The internal power supply may enter current limiting. Extended operation above maximum can damage voltage regulators and capacitors.

DC vs AC Auxiliary Supply

The test standards differ between the two.

DC auxiliary supply characteristics

• Typical source: Station battery (lead-acid, Ni-Cd, or Li-ion) with continuous float charger
• Voltage variation: From approximately 90% of nominal (end-of-discharge) to 110% of nominal (high float charge)
• AC ripple: From the battery charger, typically 1–5% RMS on a healthy charger, higher on a degraded charger
• Applicable EMC tests: IEC 61000-4-29 (dips/interruptions/variations) and IEC 61000-4-17 (ripple)

AC auxiliary supply characteristics

• Typical source: UPS or dedicated AC control bus
• Voltage variation: Depends on UPS quality — typically ±5% for modern online UPS systems
• Power quality issues: Harmonic distortion, voltage sag/swell, frequency variation
• Applicable EMC tests: IEC 61000-4-11 (dips/interruptions/variations)

The Voltage Tolerance Test

This is the core commissioning test for the aux supply circuit.

Test objective

Verify that the relay operates correctly across the full rated aux voltage range declared by the manufacturer.

Procedure

1. Baseline at rated voltage. Apply the nominal voltage (220V DC or AC). Verify:
   • The relay powers up normally
   • The display initializes correctly
   • Protection elements are enabled
   • Communications are active
   • Output contacts are in their correct quiescent state
2. Test at minimum rated voltage. Reduce the aux supply to the declared minimum (typically 80% — 176V for a 220V relay). Hold for at least 30 seconds. Verify continued operation and no spurious alarms.
3. Test at maximum rated voltage. Increase the aux supply to the declared maximum (typically 120% — 264V for a 220V relay). Hold for at least 30 seconds. Verify continued operation with no overvoltage alarms or abnormal heating.
4. Return to rated voltage and verify normal operation.

Required equipment

• Variable DC power supply (for DC testing), minimum 0–300V DC at 1–2A continuous
• Variac or variable AC source (for AC testing), 0–280V AC
• Digital voltmeter for accurate voltage measurement
• Secondary injection set for functional verification at each voltage point

AC Ripple on DC Supply (IEC 61000-4-17 / IEC 60255-26)

This is one of the most important — and most often skipped — commissioning verifications.

Why it matters

A station battery is float-charged continuously. The battery charger is typically a full-wave rectifier that produces DC with superimposed AC ripple at 100 Hz (for 50 Hz mains) or 120 Hz (for 60 Hz mains). On a new, healthy charger, the ripple is small. As the charger ages — specifically, as its smoothing capacitors dry out — the ripple increases.

The relay’s internal power supply is designed to reject ripple up to a specified level. Beyond that level, the relay may misoperate.

What the standards require

IEC 61000-4-17 is the basic test method for ripple on DC input power ports. It specifies the test waveform (half-wave or full-wave rectified AC superimposed on the DC supply) and the test levels as a percentage of the DC voltage.

IEC 60255-26 (the EMC product standard for protection relays) references IEC 61000-4-17 with specific levels appropriate for protection equipment. The typical test level for protection relays is 12% peak-to-peak ripple superimposed on the rated DC voltage, with the relay required to continue operating correctly (performance criterion A — no observable effect).

For a 220V DC relay, this means the relay must function correctly with about 26V peak-to-peak ripple on the 220V bus.

Field verification of ripple

During commissioning and periodic maintenance, you can measure actual ripple on the station battery bus:

1. With a true-RMS multimeter in AC mode, measure across the battery terminals
2. The AC reading represents the ripple voltage (approximate RMS of the ripple component)
3. Multiply by approximately 2.8 to estimate peak-to-peak ripple
4. Convert to percentage: (peak-to-peak ripple ÷ DC voltage) × 100

Typical acceptable field values:

• Below 2% peak-to-peak: Excellent — new or well-maintained charger
• 2–5% peak-to-peak: Normal range for in-service chargers
• 5–10% peak-to-peak: Investigate — likely capacitor aging
• Above 10% peak-to-peak: Charger maintenance required

Note that the 12% figure in IEC 60255-26 is a type test stress level (ensuring equipment can withstand poor-quality supplies). In actual operation, you want ripple well below that — typically under 5%.

Voltage Dips, Interruptions, and Variations

These tests verify the relay’s behavior during brief disturbances on the aux supply. The applicable standard depends on whether your aux supply is AC or DC.

For 220V DC aux supply: IEC 61000-4-29

IEC 61000-4-29 specifies test levels and durations for DC input power ports. The preferred test levels and durations are:

Voltage dips (residual voltage during the dip):

• 40% of U_T (rated voltage) — duration: 10 ms, 30 ms, 100 ms, 300 ms, or 1 s
• 70% of U_T — duration: 10 ms, 30 ms, 100 ms, 300 ms, or 1 s

Short interruptions (0% of U_T — complete loss):

• Duration: 1 ms, 3 ms, 10 ms, 30 ms, 100 ms, 300 ms, or 1 s
• Tested under high-impedance AND/OR low-impedance source conditions

Voltage variations (slow transitions to different levels):

• 80% and 120% of U_T, OR 85% and 120% of U_T
• Duration: 100 ms, 300 ms, 1 s, 3 s, or 10 s

For each test, the EUT is subjected to three events at minimum 10-second intervals. The product committee (IEC 60255-26 for relays) specifies which combination of levels and durations applies and which performance criterion (A, B, C, or D) must be met.

For 220V AC aux supply: IEC 61000-4-11

IEC 61000-4-11 specifies test levels and durations for AC input power ports (50/60 Hz):

Voltage dips:

• 0% of U_T for 0.5 cycle (10 ms at 50 Hz / 8.3 ms at 60 Hz) — mandatory for all equipment
• 0% of U_T for 1 cycle (20 ms at 50 Hz / 16.7 ms at 60 Hz)
• 40% of U_T for 10/12 cycles (200 ms)
• 70% of U_T for 25/30 cycles (500 ms)
• 80% of U_T for 250/300 cycles (5 s)

Short interruptions:

• 0% of U_T for 250/300 cycles (5 s)

Again, the product committee assigns specific performance criteria (A, B, C, D) to each test combination.

Performance criteria explained

Both standards use the same four-level classification:

Criterion	Meaning
A	Normal performance — no observable effect
B	Temporary loss with automatic recovery when disturbance ends
C	Temporary loss requiring operator intervention to recover
D	Permanent loss — damage, data loss, or loss of function

For a critical protection relay, aim for Criterion A through the durations expected in your installation and Criterion B at worst for longer interruptions. A relay that shows Criterion C or D under normal substation disturbances is inadequate for protection duty.

For a detailed breakdown of IEC 61000-4-11 and IEC 61000-4-29, see our separate article: IEC 61000-4-11 & IEC 61000-4-29: Voltage Dip Immunity Testing Explained.

Commissioning Test Procedure for 220V Aux Supply

Here’s the practical sequence I use during substation commissioning for 220V DC aux supply on protection relays.

Step 1: Verify the nominal supply

Before any testing, verify the nominal 220V DC at the relay input using a true-RMS multimeter. Expected value: 216–228V DC when the battery is at float charge. AC ripple: below 5% peak-to-peak (roughly 4V AC RMS on a 220V bus).

If actual voltage is outside this range, fix the battery/charger before testing the relay.

Step 2: Dielectric and insulation tests

Per IEC 60255-27 and IEC 60255-5:

• Disconnect the relay’s aux supply terminals from the external wiring
• IR test at 500V DC between the aux supply input and ground, per IEC 60255-5 (minimum 100 MΩ for new relays)
• Dielectric test at 2 kV AC for 1 minute between aux supply input and ground (if required by commissioning specification and only for new/re-tested equipment)

Important: Disconnect the relay from the aux supply and short the aux terminals together during these tests. Applying 500V DC to live aux inputs will damage the relay’s internal power supply.

Step 3: Voltage tolerance test

With a controlled variable DC supply (not the station battery):

• Set to nominal (220V). Verify normal relay operation.
• Reduce to 176V (80%). Verify continued operation.
• Increase to 264V (120%). Verify continued operation.
• Return to nominal. Verify operation.

Step 4: Voltage dip and interruption test (for critical installations)

These tests verify the manufacturer’s EMC compliance claims in your actual installation — typically done only for protection relays on Category A (critical) circuits. Using a test set that can produce controlled DC voltage dips per IEC 61000-4-29:

• Apply a dip to 70% of nominal for 100 ms. Verify performance criterion A (no trip output, no reset).
• Apply a dip to 40% of nominal for 100 ms. Verify performance criterion per manufacturer’s declaration.
• Apply a complete interruption of 30 ms. Verify the relay rides through without reset.
• Apply a complete interruption of 1 second. Verify clean recovery without loss of settings.

For most distribution feeder relays, this level of testing is beyond the scope of field commissioning — you trust the IEC 60255-26 type test certificate. For main protection (bus differential, transformer differential, generator protection), specifying relays with documented Criterion A performance through expected disturbances is appropriate.

Step 5: Ripple verification

Reconnect the relay to the station battery bus. Measure actual DC voltage and AC ripple at the relay terminals. Document the readings as baseline.

Step 6: Functional verification

With all voltage tolerance tests complete, perform the standard protection commissioning tests (secondary injection, trip verification, communication check) at nominal voltage.

Maintenance Testing of the Station Battery

The aux supply is only as reliable as the battery that feeds it. Battery maintenance is beyond the scope of this article, but the essentials for a 220V DC system:

Annual battery testing (IEEE 450 for lead-acid, IEEE 1188 for VRLA)

• Individual cell voltage — verify balanced within ±0.02V per cell at float charge
• Battery impedance — using a dedicated battery impedance tester (like the Megger BITE series). Rising impedance indicates dying cells.
• Ripple measurement at the battery terminals — should be below 5% peak-to-peak
• Load test (less frequent — every 3–5 years) — discharge the battery through a controlled load and verify capacity

Signs of aux supply problems at the relay

If relays are experiencing mysterious resets, logic errors, or communication failures, suspect the aux supply before suspecting the relay:

• Measure DC voltage and ripple at the relay terminals (not at the battery)
• Check for voltage drop across long wiring runs (high current draws can cause 5–10V drop on older installations)
• Check ground connections — a floating aux supply ground can cause strange behavior
• Verify the supply polarity at the relay — reversed polarity damages some relays, though modern ones have reverse-polarity protection

Common Problems with 220V Aux Supply

Aged battery charger producing excessive ripple. The single most common aux supply issue. Replace capacitors in the charger’s output filter, or replace the charger.

Voltage drop on long wiring runs. Relays in remote panels may see 200V at their terminals when the battery is at 220V. Size aux supply wiring for minimal voltage drop, or use local regulation.

Parallel relays on a single aux supply branch. A relay drawing 5A briefly (during LCD backlighting or LED test, for example) can pull the voltage down enough to reset another relay on the same branch. Provide each relay its own fuse and wire gauge adequate for startup current.

Aux supply ground loops. The aux supply negative terminal may be grounded at multiple points in the substation, creating circulating currents. Follow the manufacturer’s grounding diagram exactly.

Wrong polarity on DC aux input. Modern relays are polarity-protected; older relays are not. Always verify polarity before energizing.

Mixing 220V DC and 220V AC inputs. Most modern numerical relays accept both, but some older relays are rated only for one. Verify the relay’s aux input rating before connecting.

FAQ

Key Takeaways

• 220V aux supply typically means either 220V DC from a station battery or 220V AC from a UPS-backed control bus. The applicable EMC test standards differ.
• For DC aux supply: IEC 61000-4-29 covers voltage dips/interruptions/variations. IEC 61000-4-17 covers AC ripple. IEC 60255-26 specifies which levels apply to protection relays.
• For AC aux supply: IEC 61000-4-11 covers voltage dips/interruptions/variations.
• Rated voltage tolerance is typically 80%–120% of nominal (176V to 264V for a 220V relay). The relay must operate correctly across this range.
• AC ripple on DC is NOT tested by IEC 61000-4-29 — it’s covered by IEC 61000-4-17, a separate standard. A common confusion point.
• IEC 61000-4-29 dip test levels: 40% and 70% of U_T, durations 10 ms to 1 s. Interruption levels: 0% U_T, durations 1 ms to 1 s, tested under both high-impedance and low-impedance source conditions.
• Performance criteria matter: Criterion A (no effect) vs B (automatic recovery) vs C (operator reset needed) determines whether your relay keeps protecting during the disturbance.
• Most aux supply problems trace back to the battery charger (excessive ripple), undersized wiring (voltage drop), or aging batteries (high impedance). Check these before suspecting the relay.
• Modern wide-range relays (88–300V DC / 110–250V AC) provide significant margin and are generally more forgiving of aux supply variations than older fixed-voltage relays.

Standards Referenced in This Article

Standard	Key Content
IEC 60255-1:2022	Common requirements — rated auxiliary voltage, tolerances, test reference conditions
IEC 60255-5:2000	Insulation coordination — IR test at 500V DC, dielectric test at 2 kV AC
IEC 60255-26	EMC requirements for protection relays — specifies which 61000-4-x tests apply and at what levels
IEC 60255-27:2023	Product safety — dielectric, touch voltage, protective bonding
IEC 61000-4-11:2004+AMD1:2017	Voltage dips, interruptions, and variations on AC power ports
IEC 61000-4-17	AC ripple on DC input power port immunity
IEC 61000-4-29:2000	Voltage dips, interruptions, and variations on DC power ports
IEEE 450	Maintenance, testing, and replacement of vented lead-acid batteries
IEEE 1188	Maintenance, testing, and replacement of VRLA batteries