Protection relays sit at the heart of every substation and industrial power system. They make life-or-death decisions in milliseconds — tripping breakers to isolate faults before equipment is destroyed or people are hurt. If the insulation inside a protection relay fails, the relay itself becomes the fault. And that can take out the protection for an entire feeder, bus, or transformer.
IEC 60255-5 is the standard that defines insulation coordination for these devices — the test voltages, impulse withstand levels, and minimum insulation resistance values that every protection relay must meet. This article breaks down the standard’s requirements with the specific clause numbers and test values from the actual document.
Note: IEC 60255-5:2000 has been superseded by IEC 60255-27:2013 for new designs. However, the 2000 edition remains the reference for the vast majority of installed protection relays worldwide and is still widely used in commissioning and maintenance testing. The core insulation requirements remain consistent between editions.
Table of Contents
What IEC 60255-5 Covers
IEC 60255-5 is titled “Electrical relays — Part 5: Insulation coordination for measuring relays and protection equipment — Requirements and tests.”
It covers three main areas (Clause 1):
Definitions — Terms related to insulation voltage ratings, creepage, clearances, and pollution degrees.
Design guidance — How to select clearances and creepage distances for relay insulation.
Test requirements — The voltage tests and insulation resistance measurements that verify the insulation meets its rated capability.
The standard applies to all measuring relays and protection equipment — numerical, static, and electromechanical. It also covers associated auxiliary devices like shunts, series resistors, and current/voltage transformers used with the relays.
Scope and Applicability
Per Clause 1, IEC 60255-5 applies to:
- Equipment with rated AC voltage up to 1,000V (frequency up to 65 Hz)
- Equipment with rated DC voltage up to 1,500V
- Installation at altitudes up to 2,000 m above sea level
For altitudes above 2,000 m, the standard provides correction factors in Table B.1 (Annex B) to account for reduced air pressure and its effect on dielectric withstand.
The standard is designed to be read alongside IEC 60664-1 (Insulation coordination for equipment within low-voltage supply systems), which provides the underlying framework.
Insulation Resistance Test Requirements
The test — Clause 6.2.2
The insulation resistance measurement is a type test. It can also be performed as a routine or commissioning test if agreed between manufacturer and user.
Key specifications from Clause 6.2.2:
| Parameter | Requirement |
|---|---|
| Test voltage | 500V DC ± 10% |
| Minimum reading time | At least 5 seconds after applying voltage |
| Minimum IR for new relays | ≥ 100 MΩ |
| Test type | Type test (routine by agreement) |
Test connections
The IR measurement is made:
- Between each circuit and the exposed conductive parts (terminals of each independent circuit connected together)
- Between independent circuits (terminals of each independent circuit connected together)
- Optionally: between open contacts (by agreement)
Circuits having the same rated insulation voltage may be connected together when being measured to exposed conductive parts.
Important caveat — EMC components
The standard notes that insulation under test may be paralleled by EMC suppression components or other functional components whose insulation resistance is below 100 MΩ. This is common in modern numerical relays that have capacitors to earth for EMC compliance.
In such cases, the manufacturer must verify that these components are not damaged by the test procedure and can still maintain isolation against hazardous voltages.
What this means in practice
When I commission protection relays, I perform the 500V DC IR test between each independent circuit group and the relay housing (exposed conductive parts). On a typical numerical relay, the independent circuits are:
- CT input circuits
- VT input circuits
- Auxiliary supply (DC power)
- Binary output (trip/close contacts)
- Communication ports
I connect all terminals of each circuit group together, then measure to ground. Each should read well above 100 MΩ. On new relays, I typically see readings in the GΩ range.
Impulse Voltage Withstand Test (5 kV)
The standard test — Clause 6.1.3
This is the most important insulation test for protection relays. It verifies that the relay can survive voltage transients from lightning and switching operations in the substation.
Key specifications from Clause 6.1.3:
| Parameter | Requirement |
|---|---|
| Waveform | 1.2/50 μs (standard lightning impulse) |
| Standard test voltage | 5 kV for relays on CT/VT/battery circuits |
| Source impedance | 500 Ω ± 10% |
| Output energy | 0.5 J ± 10% |
| Test leads | Maximum 2 m length |
| Test type | Type test (routine by agreement) |
Why 5 kV?
The standard explains this in the Introduction. Relays connected directly to current and voltage transformers or station battery supplies operate in Overvoltage Category III environments, where transient overvoltages from the substation environment are significant.
Rather than requiring a range of test voltages for different installations, the standard rationalizes to a single 5 kV impulse test voltage. This single value covers the normal case regardless of altitude (sea level to 2,000 m) and simplifies testing.
Test procedure (Clause 6.1.3.4)
The impulse is applied:
- Between each circuit (or group of circuits) and exposed conductive parts
- Between independent circuits (terminals connected together)
- Between terminals of a given circuit (by agreement only)
Circuits not involved in the test are connected together and to earth.
For verification of clearances: Minimum 3 impulses of each polarity, at least 1 second between impulses.
For verification of solid insulation: 5 impulses of each polarity, with the waveform of each impulse recorded.
Acceptance criteria (Clause 6.1.3.5)
No disruptive discharge (sparkover, flashover, or puncture) during the test. Partial discharges in clearances that don’t result in breakdown are disregarded. After the test, the relay must comply with all relevant performance requirements.
Repeat testing (Clause 6.1.3.6)
For relays in new condition, impulse tests may be repeated if necessary. The repeat test voltage is 0.75 × the original value (i.e., 3.75 kV for a 5 kV rated relay).
Dielectric (AC High-Voltage) Test
Requirements — Clause 6.1.4, Table 6
The dielectric test (AC power frequency high-voltage test) verifies the solid insulation’s long-term durability. It’s a type test and also a routine test.
AC test voltages from Table 6:
| Rated Insulation Voltage | AC Test Voltage |
|---|---|
| Up to 63V | 0.5 kV |
| 125V – 500V | 2.0 kV |
| 630V | 2.3 kV |
| 800V | 2.6 kV |
| 1,000V | 3.0 kV |
For circuits energized directly via instrument transformers, the test voltage must be at least 2 kV — regardless of the rated insulation voltage.
Test method (Clause 6.1.4.4)
The voltage is initially set to less than 50% of the specified value, then raised smoothly to full test voltage. It’s held for 1 minute, then reduced smoothly to zero.
For routine tests: The voltage may be held for only 1 second, but at 110% of the value specified in Table 6.
The test voltage must be substantially sinusoidal, at a frequency between 45 Hz and 65 Hz. Alternatively, a DC voltage of 1.4 × the AC value may be used by agreement.
Acceptance criteria (Clause 6.1.4.5)
No breakdown or flashover shall occur during the test. The standard notes that EMC capacitors to earth will increase the test current, making breakdown detection difficult. This can be overcome by using DC test voltage (√2 × Vrms) or by measuring only the AC resistive current.
Overvoltage Categories Explained
IEC 60255-5 defines four overvoltage categories (Clause 4.2.2.1) that determine the rated impulse voltage for relay circuits:
Category I — Special measures taken to limit transients. Well-protected electronic circuits. Lowest impulse withstand requirement.
Category II — Relay circuits not directly connected to instrument transformers. Short leads, good screening. Moderate impulse withstand.
Category III — The standard category for most protection relay applications. Applies when:
- Auxiliary supply connected to station battery with long leads
- Input circuits connected directly to CTs and VTs
- Output circuits connected to loads by long leads
Category IV — Relays subjected to very high transient levels, e.g., directly connected to primary circuits without proper shielding.
Rated impulse voltages — Table 2
For Category III at 300V line-to-neutral (the standard case for CT/VT/battery circuits):
| Overvoltage Category | Rated Impulse Voltage |
|---|---|
| Category I | 1,500V |
| Category II | 2,500V |
| Category III | 4,000V |
| Category IV | 6,000V |
The rationalized 5 kV test voltage exceeds even the Category III requirement of 4 kV, providing additional margin.
Rated Insulation Voltages
Table 1 (Clause 4.2.1.1) provides the standard rated insulation voltages matched to supply system nominal voltages:
| Supply System Nominal Voltage | Rated Insulation Voltage |
|---|---|
| 30V | 32V |
| 110V – 127V | 125V |
| 208V – 240V | 250V |
| 380V – 480V | 500V |
| 660V – 720V | 800V |
| 960V – 1,000V | 1,000V |
Critical rule: For relays directly energized via instrument transformers or connected to station battery supply, the rated insulation voltage must not be below 250V — even if the actual circuit voltage is lower.
Pollution Degrees
The standard defines four pollution degrees (Clause 4.4) that affect creepage distance requirements:
Degree 1 — No pollution or only dry, non-conductive pollution. Special sealed environments only.
Degree 2 — Non-conductive pollution, but occasional condensation possible. Minimum for utility and industrial relay installations.
Degree 3 — Conductive pollution or dry pollution that becomes conductive due to condensation.
Degree 4 — Persistent conductivity from conductive dust, rain, or snow.
Protection relays for utility or industrial power systems must be designed for at least Pollution Degree 2.
Clearances and Creepage Distances
Clearances — Table 3
Minimum clearances in air are based on the rated impulse voltage and pollution degree. For the standard 5 kV impulse test voltage:
| Pollution Degree | Minimum Clearance |
|---|---|
| Degree 1 | 4 mm |
| Degree 2 | 4 mm |
| Degree 3 | 4 mm |
| Degree 4 | 4 mm |
At 5 kV, the clearance is 4 mm regardless of pollution degree.
Creepage distances — Table 4
Creepage distances depend on the rated insulation voltage, pollution degree, and insulating material group. For a relay with 250V rated insulation voltage at Pollution Degree 2:
| Material Group | Minimum Creepage Distance |
|---|---|
| Group I | 1.25 mm |
| Group II | 1.8 mm |
| Group III | 2.5 mm |
For 500V rated insulation voltage at Pollution Degree 2:
| Material Group | Minimum Creepage Distance |
|---|---|
| Group I | 2.5 mm |
| Group II | 3.6 mm |
| Group III | 5.0 mm |
Test Sequence and Order
Per Clause 6.1.6, insulation tests must be performed in this order:
- Impulse voltage withstand test (Clause 6.1.3) — first
- Dielectric test (AC power frequency high-voltage test, Clause 6.1.4) — second
The impulse test comes first because it applies the highest peak voltage and is the more stressful test. The dielectric test follows to verify sustained voltage withstand.
Routine vs Type Tests
| Test | Type Test | Routine Test |
|---|---|---|
| Impulse voltage withstand (5 kV) | Yes | By agreement |
| Dielectric test (2 kV AC) | Yes | Yes |
| Insulation resistance (500V DC) | Yes | By agreement |
| Creepage distance measurement | Yes | No |
The dielectric test is always a routine test — it’s performed on every relay manufactured. The impulse test and IR measurement are type tests by default but can be added as routine tests by agreement between manufacturer and user.
How I Test Protection Relay Insulation in the Field
After 12 years of commissioning protection systems on 66 kV and 220 kV substations, here’s the procedure I follow:
Before testing
- De-energize the relay and disconnect all external wiring from the relay terminals
- Remove communication cables (fiber and copper)
- Disconnect any surge protection devices connected to the relay terminals
- Verify the relay is completely isolated
IR test (500V DC)
- Connect megger LINE lead to the circuit group under test (all terminals of that group connected together)
- Connect megger EARTH lead to the relay housing / ground terminal
- Apply 500V DC for a minimum of 60 seconds (I go longer than the standard’s 5 seconds to get a stable reading)
- Record the reading
- Repeat for each independent circuit group
What I look for
- New relay: >1 GΩ is typical. Below 100 MΩ on a new relay is a problem.
- Relay in service for several years: anything above 100 MΩ is acceptable.
- Below 50 MΩ: investigate. Could be moisture ingress, contaminated terminals, or internal component degradation.
- Compare between circuit groups: all should be in the same range. One significantly lower reading suggests a localized problem in that circuit.
When I do NOT apply the 5 kV impulse test in the field
The impulse test is a factory type test. I don’t perform it during routine maintenance because:
- It requires specialized impulse test generators with precisely controlled 500 Ω impedance and 0.5 J energy
- There’s a risk of damaging EMC components inside the relay
- The 500V DC IR test and 2 kV AC dielectric test are sufficient for verifying field insulation integrity
The dielectric test at 2 kV AC (or its DC equivalent at 2.8 kV) is what I use for commissioning verification.
Quick Reference Table
| Test | Clause | Test Voltage | Acceptance | Test Type |
|---|---|---|---|---|
| IR measurement | 6.2.2 | 500V DC ±10% | ≥ 100 MΩ (new) | Type (routine by agreement) |
| Impulse withstand | 6.1.3 | 5 kV (1.2/50 μs) | No discharge | Type (routine by agreement) |
| Impulse — repeat test | 6.1.3.6 | 0.75 × original | No discharge | — |
| Dielectric (AC) | 6.1.4 | 2 kV (for 125–500V rated) | No breakdown in 1 min | Type + Routine |
| Dielectric — routine | 6.1.4.4 | 110% of Table 6 value | No breakdown in 1 sec | Routine |
| Impulse generator | 6.1.3.2 | — | 500 Ω, 0.5 J, 1.2/50 μs | — |
FAQ
Is IEC 60255-5 still valid?
IEC 60255-5:2000 was superseded by IEC 60255-27:2013 for new relay designs. However, the vast majority of installed relays worldwide were designed and tested to IEC 60255-5. The core insulation requirements — 5 kV impulse, 2 kV dielectric, 100 MΩ minimum IR — remain consistent. For maintenance testing of existing equipment, the original standard’s values still apply.
Why is the impulse test at 5 kV and not higher?
The 5 kV impulse test covers Overvoltage Category III at up to 300V line-to-neutral, which represents the standard substation environment for relays connected to CTs, VTs, and battery supplies. The generator impedance is 500 Ω (matching substation wiring impedance), and the energy is limited to 0.5 J to avoid damaging relay components.
Can I use a standard megger for the 500V DC IR test?
Yes. Any insulation resistance tester with a 500V DC output and a range up to at least 100 MΩ will work. Most modern megohmmeters far exceed this capability.
What’s the difference between the impulse test and the dielectric test?
The impulse test (5 kV, 1.2/50 μs) simulates fast transients like lightning surges. It verifies clearances and solid insulation against brief, high-peak-voltage events. The dielectric test (2 kV AC, 1 minute) verifies the insulation can withstand sustained overvoltages and proves long-term durability. Both are needed for a complete insulation assessment.
Do I need to test every relay in a substation?
For commissioning: yes. Test every relay before energization. For routine maintenance: focus on relays in critical applications (bus protection, transformer differential, line distance) and any relay showing signs of environmental exposure (humidity, dust, corrosive gases).
Key Takeaways
- IEC 60255-5 requires a minimum IR of 100 MΩ at 500V DC for new protection relays (Clause 6.2.2).
- The standard impulse test is 5 kV (1.2/50 μs) with 500 Ω impedance and 0.5 J energy (Clause 6.1.3).
- The dielectric test is 2 kV AC for 1 minute for relays with rated insulation voltage of 125V–500V (Table 6).
- Most protection relay installations fall under Overvoltage Category III and Pollution Degree 2.
- The dielectric test is always a routine test. The impulse test and IR measurement are type tests unless otherwise agreed.
- For field commissioning, the 500V DC IR test and 2 kV dielectric test are the practical minimum.
- EMC components may affect IR readings on modern numerical relays — the standard addresses this in Clause 6.2.2.
Standards Referenced in This Article
| Standard | Edition | Key Content Used |
|---|---|---|
| IEC 60255-5 | Ed. 2.0 (2000) | Full standard: Clauses 1, 4, 5, 6.1.3, 6.1.4, 6.2.2; Tables 1–7 |
| IEC 60255-27 | Ed. 1.0 (2013) | Successor standard (referenced for context) |
| IEC 60664-1 | — | Insulation coordination framework (referenced by IEC 60255-5) |
| IEC 60060-1 | — | High-voltage test techniques (impulse generator requirements) |
| IEC 60085 | — | Thermal evaluation of insulation (referenced for material classification) |