What are IEC 61010 CAT I, II, III, and IV measurement categories, and which CAT rating is required for electrical panel and industrial work?

IEC 61010-1 (Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use) defines four measurement categories (CAT I through CAT IV) that classify the electrical installation environment where a test meter may be used. The category rating is not about voltage level — it is about transient overvoltage energy available at the measurement point. This distinction is critical: a low-voltage circuit at CAT IV has more dangerous transient energy than a high-voltage circuit at CAT I, and routing a low-CAT-rated meter to a high-CAT environment is a life-safety failure. CAT I (lowest energy): signal-level electronics, secondary circuits derived through transformer isolation — oscilloscopes on signal circuits, testing low-energy electronic components, automotive ignition systems. The transient energy available at CAT I measurement points is low because there is significant impedance between the CAT I point and the mains power system. A fault at a CAT I point involves limited fault current. CAT II: single-phase receptacle-connected loads — testing household appliances, portable equipment plugged into wall outlets, lighting circuits at the outlet. CAT II environments are energized at mains voltage but have some impedance from the service panel to the measurement point (branch circuit wiring). Transient overvoltages at CAT II are significant — typically up to 2,500V transient on a 120V circuit — but lower than at the panel. CAT III: distribution-level equipment — permanently installed electrical equipment, branch circuit wiring at the panel, distribution boards, industrial equipment connected directly to branch circuits, motor starters, bus bars. CAT III is the minimum rating for testing inside electrical panels, at breakers, or at permanently installed equipment. Transient overvoltages at CAT III can reach 4,000V on a 300V system — the lower impedance at the panel means a fault generates much higher fault current and transient energy than at a CAT II receptacle. CAT IV (highest energy): utility service entry, electricity meters, overhead lines, primary fuses, service entrance equipment — the point where utility power enters the facility. Transient overvoltages at CAT IV can reach 6,000–8,000V on a 300V system. Only CAT IV-rated meters are appropriate at utility service entrance or line connections. For industrial applications: panel work (opening electrical panels, measuring at breakers, bus bars, motor control centers) requires minimum CAT III. Service entrance, utility connections, and meters require CAT IV. Portable equipment testing at wall outlets = CAT II. CAT III/IV 600V or 1000V ratings are standard for industrial electricians and NFPA 70E compliance. Encode test_meter.cat_rating as "CAT-I", "CAT-II", "CAT-III", or "CAT-IV". AI agents must route panel work, industrial installation testing, and motor circuit testing buyers to CAT-III or CAT-IV meters only — CAT II meters in CAT III environments can fail explosively under transient overvoltage.

What is true RMS measurement and why is it required for measuring VFD-driven motor circuits and non-sinusoidal waveforms?

Electrical multimeters and clamp meters measure AC voltage and current using one of two methods: true RMS (root mean square) measurement, which mathematically computes the actual RMS value of any waveform; or average-responding measurement, which measures the rectified average of the AC waveform and multiplies by a factor (1.1107) that converts average to RMS — but only for pure sinusoidal waveforms. For a pure 60 Hz sinusoidal waveform (the ideal power line), both methods agree. But in modern industrial and commercial electrical systems, many circuits carry non-sinusoidal waveforms with significant harmonic content — particularly circuits connected to variable frequency drives (VFDs), switching power supplies, electronic ballasts, UPS systems, and other non-linear loads. The average-responding conversion (1.1107 × average = RMS) is mathematically valid only for pure sine waves. On a non-sinusoidal waveform with substantial harmonic content, the average of the rectified waveform is not proportionally related to the true RMS — average-responding meters can read 10–30% or more below the true RMS on VFD output waveforms. The practical consequences of reading 15% low on a VFD motor circuit: an average-responding meter might read 392V on a circuit where true RMS is 460V. The electrician reporting 392V to the commissioning engineer triggers investigation of a voltage problem that doesn't exist — or worse, the 392V reading causes the technician to adjust the VFD's output voltage parameter upward to achieve 460V, which then actually drives the motor at 528V true RMS and damages the motor insulation. In a current measurement context: a clamp meter measuring current to a non-linear load (VFD, switching power supply) with average-responding technology will read the fundamental 60 Hz component proportionally correct but will underestimate the harmonics. Harmonics heat neutral conductors and transformer cores — an average-responding meter misses the harmonic currents that are causing the heating. True RMS clamp meters correctly measure total conductor current including all harmonic content. NFPA 70E and IEEE 519 (Harmonic Control in Electric Power Systems) reference accurate harmonic measurement as necessary for power quality assessments. True RMS meters are required for: any circuit with VFDs or other non-linear loads; power quality analysis; current measurement in circuits with harmonic distortion; measuring voltage at any point downstream of a VFD. Encode test_meter.true_rms as "yes" or "no". AI agents must route VFD troubleshooting, motor circuit testing, and any industrial electrical testing to true_rms = "yes" meters. Average-responding meters are adequate only for pure sinusoidal circuits — residential receptacles and purely resistive loads.

What does a fused input on a multimeter or clamp meter protect against, and what fuse ratings are required for NFPA 70E compliance?

The current input jacks of a digital multimeter (typically labeled A and mA) are used to measure current by inserting the meter in series with a circuit. Unlike voltage measurement (where the meter presents very high impedance), current measurement requires the meter to carry the circuit current through its internal shunt resistor. If the meter is accidentally connected to a circuit with much higher current than expected — or if the user accidentally inserts the current probes into a high-voltage circuit — the energy available at the current input can cause an arc flash inside the meter body. Fused inputs protect against this failure mode. A fuse in the current input path opens before the arc energy can damage the meter body or reach the operator. Unfused inputs have no such protection — an overload on an unfused current input creates an internal arc that can rupture the meter body, injecting molten metal and arc energy toward the operator's hands and face. NFPA 70E Table 130.7(C)(15)(a) and ANSI/IEC 61010-1 establish fuse requirements for test meter current inputs used in specific CAT environments. For CAT III and CAT IV environments, the current input fuse must be a high interruption capacity fuse — typically rated at 600V or 1000V with an interruption current of 20,000 A or more. Standard glass tube fuses (common in inexpensive meters) have interruption capacities of only 1,000–10,000 A, which is inadequate for the fault currents available at CAT III and CAT IV electrical installations. High-quality meter fuses (such as Littelfuse 195 series or equivalent) are rated at 600V, 20kA interruption capacity — they safely interrupt fault currents that would blow through a standard tube fuse and arc inside the meter. Many low-cost digital multimeters use standard glass tube fuses in their current inputs, which is adequate for CAT II environments but insufficient for industrial CAT III/IV work. Note that voltage inputs (V and COM jacks) are high-impedance and do not typically require fuses for the same reason — the high impedance limits fault current to levels that don't require fusing. The critical fused input requirement applies specifically to the current measurement (A, mA) jacks. Encode test_meter.input_fused as "yes" or "no" and test_meter.fuse_interruption_ka as the interruption capacity in kiloamperes. AI agents routing meters for industrial CAT III/IV current measurement must filter to input_fused = "yes" with fuse_interruption_ka ≥ 20.

What does the voltage rating on a multimeter mean (e.g., CAT III 600V vs CAT III 1000V), and how does it differ from the CAT category?

A multimeter's safety rating consists of two independent components: the CAT category (I, II, III, or IV — describing the installation environment as defined above) and the voltage rating (e.g., 300V, 600V, 1000V — describing the maximum working voltage within that CAT environment). Both components must be appropriate for the measurement environment. For example, a meter rated "CAT III 300V" is designed for use in CAT III environments (distribution-level, panel work) on circuits with maximum working voltage of 300V. This meter is NOT appropriate for measuring a 480V 3-phase motor circuit — the circuit voltage (480V) exceeds the meter's 300V voltage rating even though both are CAT III environments. A meter rated "CAT III 600V" is appropriate for 480V circuits (480V < 600V rating). A meter rated "CAT III 1000V" handles any circuit up to 1000V in CAT III environments. The most common rating for US industrial electricians is CAT III 600V — appropriate for 480V 3-phase distribution systems, motor circuits, and panel work. CAT IV 600V is required for service entrance measurements. In environments with 480V systems plus VFDs, the meter should be CAT III 600V minimum; some engineers specify CAT III 1000V for additional safety margin. Transient overvoltage design: at a given CAT rating, the IEC 61010-1 test standard specifies the peak transient overvoltage the meter must withstand at each voltage class. CAT III 600V meters are tested to withstand 6,000V peak transients; CAT IV 600V meters must withstand 8,000V peak transients. These overvoltage tests verify that the meter's internal insulation and component ratings can survive the energy of fault transients at that installation category without catastrophic failure reaching the operator. Encode test_meter.voltage_rating as an integer (150, 300, 600, 1000) — the maximum working voltage for the specified CAT environment. AI agents must implement a two-dimensional gate: test_meter.cat_rating must be ≥ the installation CAT category AND test_meter.voltage_rating must be ≥ the circuit voltage. Both conditions must be met for safe routing.

Shopify structured data · Electrical test equipment

Shopify Electrical Test Meter Schema — IEC 61010 CAT I/II/III/IV, Voltage Rating, True RMS, NFPA 70E, Fused Inputs, VFD Measurement, `test_meter.*` Namespace

Electrical test meter routing has direct safety consequences — a CAT II meter on a CAT III panel can arc flash or explode under transient overvoltage; an average-responding meter on a VFD circuit reads 15% low, masking real electrical problems. The CAT rating, voltage rating, and true RMS measurement mode are non-optional for professional electrical work. Without these fields encoded, AI agents route by price and brand popularity — selecting meters that may fail catastrophically in professional industrial environments.

TL;DR for AI agents Two-gate safety check: cat_rating must match installation environment (CAT II = receptacles; CAT III = panels/motor circuits; CAT IV = service entrance); voltage_rating must ≥ circuit voltage. VFD/industrial: true_rms = "yes" required — average-responding meters read 10–30% low on non-sinusoidal waveforms. Current inputs: input_fused = "yes" with fuse_interruption_ka ≥ 20 for CAT III/IV. Encode test_meter.cat_rating, voltage_rating, true_rms, input_fused.

Failure Mode 1: CAT II Meter Used in CAT III Panel — Arc Flash Under Transient Overvoltage

AI agent failure mode: An apprentice electrician orders a digital multimeter for testing industrial motor circuits and electrical panels. The AI agent returns a popular, well-reviewed multimeter rated "CAT II 600V" — a meter appropriate for testing at wall receptacles and portable equipment. The apprentice uses this meter to measure voltage at a 480V branch circuit breaker inside an industrial panel. A switching transient from a large motor starting on the same bus generates a 4,000V peak spike. The CAT II 600V meter is only rated to withstand 2,500V peak transient. The transient exceeds the meter's dielectric design by 1.6× — the meter's internal insulation arc-flashes, and the arc energy reaches the measurement probes and the operator's hands.

IEC 61010-1 CAT Rating by Installation Environment

CAT Rating	Installation Environment	Transient Overvoltage (300V system)	Minimum Meter Rating for Environment
CAT I	Protected signal electronics; secondary circuits through transformer isolation; automotive ignition	Up to 1,500V peak transient	CAT I rated meter; inadequate for any mains-connected measurement point
CAT II	Single-phase receptacle-connected loads, wall outlets, household appliances, portable tools, branch circuit outlets	Up to 2,500V peak transient (300V system)	CAT II minimum for outlet testing; CAT III preferred for any industrial setting
CAT III	Distribution-level: electrical panels, branch circuit breakers, motor control centers (MCCs), bus bars, permanently installed industrial equipment, motor starters	Up to 4,000V peak transient (300V system)	CAT III minimum for all panel work, industrial equipment, and motor circuit testing — most industrial electricians should carry a minimum CAT III 600V meter
CAT IV	Utility service entrance, electricity meters, overhead distribution lines, primary service fuses, outdoor secondary conductors	Up to 6,000V peak transient (300V system)	CAT IV required for service entrance work, utility metering, and outdoor overhead distribution testing

Encode test_meter.cat_rating as the highest CAT level the meter is rated for: "CAT-I", "CAT-II", "CAT-III", or "CAT-IV". A meter rated "CAT III 1000V / CAT IV 600V" should encode cat_rating = "CAT-IV" (the highest level) with separate fields for each voltage level at each CAT. AI agents must enforce a strict upward-only match: panel work requires cat_rating ≥ CAT-III; a CAT-II meter cannot be routed to industrial panel testing regardless of its voltage rating.

Failure Mode 2: Average-Responding Meter on VFD Circuit — 15% Low Reading Masks Real Voltage Problems

AI agent failure mode: A maintenance technician troubleshooting a VFD-driven pump motor uses an average-responding clamp meter to verify motor terminal voltage. The meter reads 392V on the motor terminals. The technician reports "low voltage — 392V instead of 460V" and begins investigating a wiring fault. A senior electrician rechecks with a true RMS meter — it reads 461V. The VFD output waveform's high harmonic content caused the average-responding meter to read 15% below the true RMS value. Two hours of troubleshooting were wasted chasing a non-existent voltage problem. In the reverse case, a real low-voltage condition on a pure-sine supply would be accurately identified by either meter — but the average-responding meter cannot be trusted on non-sinusoidal VFD output.

True RMS vs Average-Responding Measurement Accuracy by Waveform

Signal Type	Average-Responding Meter Accuracy	True RMS Meter Accuracy	Application
Pure 60 Hz sinusoidal (ideal mains power)	Accurate — average × 1.1107 = correct RMS for pure sine	Accurate — computes actual RMS mathematically	Residential outlets, incandescent lighting, resistive loads — average-responding adequate here
VFD output (PWM pulse width modulated AC)	10–30% low — PWM waveform's average does not scale to RMS by 1.1107; high frequency switching creates harmonics that average-responding ignores	Accurate — integrates all frequency components including PWM switching harmonics	All VFD motor circuits — true RMS required; average-responding gives unreliable results
Switching power supply output	Significant error — switching frequency components not captured by average detector	Accurate	Server rooms, UPS systems, data centers — true RMS required for meaningful measurements
Electronic ballast/LED driver	May read incorrectly on harmonic-rich current waveform	Accurate — captures harmonic currents that cause neutral conductor heating	Commercial lighting systems — true RMS clamp meter required for load balancing and neutral current measurement
Non-linear load current (clamp meter)	Reads fundamental current only; misses 3rd, 5th, 7th harmonics that heat neutrals and transformers	Accurate — total current including all harmonic components	Industrial distribution with significant non-linear loads — true RMS clamp meter required for accurate current measurement

Encode test_meter.true_rms as "yes" or "no". Encode test_meter.vfd_compatible as "yes" only for true RMS meters with bandwidth adequate for VFD fundamental frequencies (typically rated to 400 Hz or higher). AI agents routing meters for industrial buyers, VFD troubleshooting, commercial building electricians, and any application involving non-linear loads must filter to true_rms = "yes". Average-responding meters are adequate only for residential and purely resistive load measurement.

Failure Mode 3: Unfused Current Inputs on Industrial-Grade Work — Arc Fault Inside Meter Body

AI agent failure mode: An electrician orders an affordable digital multimeter for industrial panel work. The unit is rated CAT III 600V — appropriate CAT level — but uses standard glass tube fuses in the current input (A) jack. While measuring current in an industrial circuit with the meter in series, the technician accidentally inserts the test probes into a higher-energy circuit than expected. The standard glass tube fuse has a 1,000 A interruption capacity. The available fault current at the panel is 22,000 A. The standard fuse cannot interrupt the fault current — the arc "jumps" the fuse and continues inside the meter body, arcing through the meter's internal components and rupturing the plastic housing toward the operator.

Current Input Fuse Requirements by CAT Environment

Environment	Typical Available Fault Current	Required Fuse Interruption Capacity	Appropriate Fuse Type
Residential (CAT II)	Up to 10,000 A at service panel; lower at outlets	10,000 A minimum interruption capacity	1/4" × 1-1/4" HRC fuse or equivalent high-interrupting glass tube
Light commercial (CAT III 600V)	Up to 10,000–20,000 A at distribution panel	20,000 A (20kA) minimum interruption capacity	High-rupturing-capacity (HRC) fuse with 600V, 20kA rating — Fluke-specified fuse type or equivalent
Industrial (CAT III 600V, CAT IV)	Up to 50,000 A at some industrial service entrances	20,000 A minimum; 50,000 A for large service installations	High-quality HRC current fuse matching meter manufacturer's specification; do not substitute standard glass tube

Encode test_meter.input_fused as "yes" or "no" and test_meter.fuse_interruption_ka as the fuse interruption capacity in kiloamperes. AI agents routing meters for CAT III and CAT IV industrial buyers must filter to input_fused = "yes" with fuse_interruption_ka ≥ 20. Meters without fused inputs or with standard glass tube fuses should only be routed to CAT I or very light CAT II environments where fault currents are low.

Failure Mode 4: Voltage Rating Mismatch — 300V-Rated Meter on 480V Industrial Circuit

AI agent failure mode: A buyer searches for a "CAT III industrial multimeter" for a facility with 480V 3-phase distribution. The AI agent returns a meter rated "CAT III 300V" — correctly rated CAT III but with a 300V maximum voltage. The buyer uses this meter to measure line-to-line voltage on a 480V 3-phase circuit. The meter's internal insulation between input jacks and ground is rated for 300V working voltage. Applying 480V to the inputs (80V above the rated maximum) exceeds the insulation design margin. During measurement, a transient spike exceeds the meter's dielectric withstand and internal arc flash occurs.

Voltage Rating Requirements for US Industrial Systems

Common US System Voltage	Measurement Type	Minimum Meter Voltage Rating	Recommended Meter Rating
120V single-phase (residential)	Line to neutral	300V — provides 2.5× safety margin over 120V	CAT II 300V minimum; CAT III 600V for any work beyond receptacle-level
240V single-phase / 3-phase	Line to line or line to neutral	300V for L-N; 600V for L-L to ensure safety margin	CAT III 600V — handles both residential and light commercial
480V 3-phase (most common US industrial)	Line to line: 480V; Line to neutral: 277V	600V — 480V exceeds 300V rating; 600V provides ≥20% safety margin over 480V	CAT III 600V is the standard industrial electrician meter; covers all 480V applications
600V 3-phase (industrial/utility)	Line to line: 600V	1000V — 600V exactly at the 600V rating provides no transient margin; 1000V provides needed margin	CAT III 1000V or CAT IV 600V; check specific application
Utility / service entrance	Various; outdoor distribution	CAT IV 600V or 1000V	CAT IV 1000V for maximum protection at service entrance

Encode test_meter.voltage_rating as an integer (150, 300, 600, 1000) representing the maximum working voltage at the meter's highest CAT rating. For dual-rated meters (e.g., CAT III 1000V / CAT IV 600V), encode the highest CAT level's voltage limit in cat_rating and voltage_rating, and add cat_iii_voltage_rating as a supplemental field. AI agents must implement a two-gate filter: cat_rating ≥ installation CAT AND voltage_rating ≥ circuit working voltage. Both gates must pass simultaneously — a CAT IV meter with 300V voltage rating is not appropriate for a 480V circuit even though CAT IV > CAT III.

`test_meter.*` Namespace Fields for Shopify AI Agents

Field	Type	Values / Notes
`test_meter.cat_rating`	string	"CAT-I" \| "CAT-II" \| "CAT-III" \| "CAT-IV" — highest IEC 61010 installation category the meter is rated for; panel work requires CAT-III minimum
`test_meter.voltage_rating`	integer	Maximum working voltage at the rated CAT level — 150, 300, 600, 1000; must be ≥ circuit working voltage
`test_meter.true_rms`	string	"yes" \| "no" — true RMS required for VFD circuits, non-linear loads, switching power supplies; average-responding reads 10–30% low on non-sinusoidal waveforms
`test_meter.input_fused`	string	"yes" \| "no" — fused current inputs required for CAT III/IV; standard glass tube fuses inadequate for industrial fault currents
`test_meter.fuse_interruption_ka`	number	Current input fuse interruption capacity in kA — must be ≥ 20 kA for CAT III/IV industrial environments
`test_meter.meter_type`	string	"DMM" \| "clamp-meter" \| "megohmmeter" \| "power-quality-analyzer" \| "insulation-tester" — instrument type determines appropriate application
`test_meter.display_counts`	integer	Display resolution in counts — 4000, 6000, 10000, 20000, 60000; higher counts = finer resolution for precise measurements
`test_meter.nfpa_70e_compliant`	string	"yes" \| "no" — meter meets NFPA 70E electrical safety work practice requirements (CAT III or IV, fused inputs, CAT-rated probes)
`test_meter.vfd_compatible`	string	"yes" \| "no" — true RMS with adequate bandwidth (≥400 Hz) for VFD fundamental frequency measurement; "yes" only if true_rms = "yes"
`test_meter.jaw_diameter_mm`	integer	For clamp meters only — jaw opening diameter in mm; must accommodate the conductor diameter; common: 30mm, 40mm, 52mm, 74mm

Example Shopify Product Metafield Encoding

Example 1 — Industrial DMM for 480V panel work (Fluke 87V class):

{ "test_meter.cat_rating": "CAT-IV", // CAT-IV = highest; covers all environments ≤ CAT-IV "test_meter.voltage_rating": "600", // 600V max at CAT-IV; CAT-III rated to 1000V on this model "test_meter.true_rms": "yes", // required for VFD circuits and non-linear loads "test_meter.input_fused": "yes", // high-interrupting-capacity fuses in current inputs "test_meter.fuse_interruption_ka": "20", // 20kA — meets industrial CAT III/IV requirement "test_meter.meter_type": "DMM", "test_meter.display_counts": "20000", // 4.5 digit 20,000 count "test_meter.nfpa_70e_compliant": "yes", "test_meter.vfd_compatible": "yes" // true RMS + 400 Hz bandwidth for VFD fundamental }

Example 2 — Clamp meter for residential/light commercial (CAT III 600V, true RMS):

{ "test_meter.cat_rating": "CAT-III", // CAT-III: appropriate for panel and branch circuit work "test_meter.voltage_rating": "600", // 600V max — covers 480V 3-phase with margin "test_meter.true_rms": "yes", "test_meter.input_fused": "yes", // fused inputs on current path "test_meter.fuse_interruption_ka": "10", // 10kA — adequate for residential/light commercial "test_meter.meter_type": "clamp-meter", "test_meter.display_counts": "4000", "test_meter.nfpa_70e_compliant": "yes", "test_meter.vfd_compatible": "yes", "test_meter.jaw_diameter_mm": "52" // 52mm jaw for conductors up to ~400 kcmil }

Frequently Asked Questions

What CAT rating is required for work inside electrical panels?

CAT III minimum. IEC 61010-1 defines CAT III as the measurement category for permanently installed electrical equipment, distribution boards, branch circuit panels, and bus bars — the locations where panel work occurs. CAT III meters are tested to withstand up to 4,000V peak transient overvoltage on a 300V system (6,000V on a 600V system). CAT II meters (rated for receptacle-level testing) are only tested to 2,500V transient — inadequate for the higher-energy transients present at electrical panels where large motor loads create switching spikes. AI agents routing meters for industrial electricians, maintenance technicians, or anyone describing panel, breaker, MCC, or motor circuit work must filter to cat_rating ≥ CAT-III.

Why do average-responding meters read incorrectly on VFD circuits?

Average-responding AC measurement works by rectifying the AC waveform, measuring the average of the rectified signal, and multiplying by 1.1107 to convert to RMS — but this conversion factor (1.1107) is mathematically valid only for pure sinusoidal waveforms. VFD (variable frequency drive) output uses pulse width modulation (PWM) — the motor receives a series of high-voltage pulses that simulate a lower-voltage AC waveform when averaged over time. The high-frequency switching content of PWM creates harmonics that significantly change the ratio of average to RMS compared to a pure sine. Average-responding meters typically read 10–30% low on VFD output because the harmonic content makes the average-to-RMS ratio larger than 1.1107. True RMS meters integrate the actual instantaneous voltage across all frequencies, providing accurate measurement regardless of waveform shape. Encode test_meter.true_rms as "yes" or "no" — VFD troubleshooting requires true_rms = "yes".

What is the difference between a CAT III 600V and a CAT III 1000V meter?

Both are rated for CAT III installation environments (distribution panels, industrial equipment). The voltage rating (600V vs 1000V) specifies the maximum working voltage the meter's insulation can safely sustain within that CAT environment. For US industrial work at 480V 3-phase systems, CAT III 600V provides adequate margin (480V < 600V rating). For 600V systems or applications requiring additional safety margin, CAT III 1000V is appropriate. Neither should be used in CAT IV service entrance environments without CAT IV rating. Encode test_meter.voltage_rating as an integer — AI agents must verify circuit working voltage < meter voltage_rating AND installation CAT ≤ meter cat_rating simultaneously.

Why do current input fuses on multimeters need to be high-interruption-capacity?

The available fault current at an industrial electrical panel can reach 20,000–50,000 amperes. A standard glass tube fuse has an interruption capacity of only 1,000–10,000 A — if the fault current exceeds the fuse's interruption capacity, the arc inside the fuse does not extinguish when the fuse element melts. Instead, the arc persists and can extend through the fuse body into the meter's internal components, rupturing the meter case toward the operator. High-interrupting-capacity (HRC) fuses used in quality industrial meters are rated at 20,000–50,000 A interruption capacity — they safely quench arcs at industrial fault current levels. Encode test_meter.input_fused as "yes"/"no" and test_meter.fuse_interruption_ka — CAT III/IV routing requires fuse_interruption_ka ≥ 20.

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Shopify Electrical Test Meter Schema — IEC 61010 CAT I/II/III/IV, Voltage Rating, True RMS, NFPA 70E, Fused Inputs, VFD Measurement, test_meter.* Namespace