Category: Standards & Ratings

Most hazardous area work in India is specified in Zones, following the IEC system. But if the end client, the licensor or the EPC contractor is American or Canadian, the specification arrives written in Classes, Divisions and Groups instead, and the two systems do not translate cleanly.

This page sets out what the North American system actually says, where it differs from Zones, and what to establish before ordering.

Class: what kind of hazard

• Class I is flammable gases and vapours. Refineries, gas plants, solvent handling, paint lines.
• Class II is combustible dust. Grain handling, sugar, flour, coal, metal powders, pharmaceutical processing.
• Class III is ignitable fibres and flyings. Textile mills, timber processing.

A plant can have areas of more than one Class, and the requirements do not substitute for each other. The query we see most often in this area is for equipment suitable for Class I and Class II together, which is common at a boundary such as a bagging line next to a solvent store.

Division: how often the hazard is present

• Division 1 means the hazardous atmosphere is present during normal operation, or frequently because of repair, maintenance or leakage.
• Division 2 means it is present only under abnormal conditions, such as a container failing.

Division 1 is the demanding case and it changes the hardware. This is roughly where the Zone system splits Zone 0 and Zone 1 from Zone 2, but it is not a clean mapping. North American Division 1 covers broadly what Zones 0 and 1 cover together, which means a Division 1 specification is not automatically satisfied by Zone 1 equipment, nor the reverse. Treat them as different systems rather than as translations.

Group: which gas or dust

This is the part most often left off an enquiry, and it is the part that decides whether a given device is acceptable.

For Class I, gases, in descending order of difficulty:

• Group A is acetylene. The most demanding, and the reason it appears in specifications at all is that nothing less will do.
• Group B is hydrogen and gases of similar ignition energy.
• Group C is ethylene and similar.
• Group D is propane, petrol vapour, natural gas and most common hydrocarbons. The large majority of refinery and petrochemical requirements.

For Class II, dusts:

• Group E is combustible metal dust, including aluminium and magnesium.
• Group F is carbonaceous dust, including coal and carbon black.
• Group G is flour, grain, starch, sugar and most plastics.

Equipment approved for Group D is not acceptable in a Group A or B area. If your specification says Class I Division 1 without naming a Group, go back and get the Group before anyone quotes, because it changes what can be supplied.

NEMA is not an IP rating

NEMA enclosure types and IP codes are different systems and the conversions people use are approximate at best.

• NEMA 4 is watertight, including hose-directed water. Roughly comparable to IP66 in ingress terms, though the test methods differ.
• NEMA 4X adds corrosion resistance, which IP does not address at all. This is why 4X appears in food, chemical and coastal specifications.
• NEMA 7 is an explosion-proof enclosure for Class I Division 1 indoor locations. This is a hazardous area rating, not an ingress one.

The important point: NEMA 4 describes ingress protection only and says nothing about hazardous area suitability. A specification asking for a Class I Division 1 NEMA 4 proximity sensor is asking for two separate things at once, and both have to be satisfied. Our note on IP ratings covers the ingress side.

How the protection is actually achieved

Two routes dominate for proximity switches.

Intrinsic safety limits the energy available in the hazardous area below what can ignite the atmosphere. For proximity switches this almost always means a NAMUR sensor paired with a barrier or isolating amplifier in the safe area. The sensor itself is a simple two-wire device with no significant stored energy. Our guide to NAMUR control units and barriers covers the loop, and the Zone 0 guide covers the IEC side of the same question.

Explosion-proof enclosure contains any ignition inside a housing strong enough that it cannot propagate out. Heavier, more expensive, and the usual approach where intrinsic safety is impractical.

Intrinsic safety is generally the better answer for sensing, because the field device stays small and the certification covers a loop you can document.

What to establish before ordering

1. Class, Division and Group from the area classification drawing, not from memory.
2. The temperature class, which limits the surface temperature the equipment may reach.
3. Which approval scheme the project accepts. UL, CSA, ATEX and IECEx are separate schemes, and a certificate under one does not automatically satisfy an inspector working to another.
4. Whether the certification must cover the loop or only the sensor, which determines whether the barrier has to be bought as a matched pair.
5. Any NEMA enclosure requirement, which is separate from all of the above.
6. Cable length and routing, because intrinsic safety calculations depend on cable capacitance and inductance.

Talking to us about it

We manufacture NAMUR inductive proximity switches in Mumbai in M12, M18 and M30 with 4 mm to 15 mm sensing, and the matching NAMUR control unit.

Because approval requirements vary so much by scheme and jurisdiction, tell us the Class, Division, Group, temperature class and the approval scheme your specification demands when you enquire. We will confirm exactly what we can supply against it and what documentation comes with it, before the purchase order rather than during inspection.

An Indian OEM exporting machinery to Europe, the Gulf or North America takes on responsibility for the compliance of everything inside the machine. The sensors are a small part of that, but they are a part that causes disproportionate trouble — usually because the paperwork was not collected at purchase and cannot easily be obtained afterwards.

This is what to collect, and when.

IEC 60947-5-2: what it actually covers

IEC 60947-5-2 is the international standard for proximity switches. It is not a safety approval and not a certification mark — it is the specification that makes one manufacturer figures comparable with another.

Among other things it defines:

• The standard target against which sensing distance is measured — material, dimensions and shape. Without this, a sensing distance figure means nothing.
• Rated, effective and assured operating distance, and the tolerances between them. These are three different numbers and the gap between them is where field failures live.
• Switching frequency and how it is measured.
• Electrical requirements — supply range, residual current, voltage drop, short-circuit behaviour.
• Marking and documentation requirements.

Accent manufactures to IEC 60947-5-2. The practical value for an exporter is that the figures on the datasheet mean the same thing an inspector expects them to mean, and that a design calculation done against them will stand up.

What to collect, and when

The timing matters more than the list. Collect at purchase, because obtaining documents for a component bought eighteen months ago is difficult and occasionally impossible.

1. Declaration of conformity. The manufacturer statement of which directives and standards the product meets. Required for the machine technical file.
2. The datasheet for the exact variant, not the series. Ratings differ between variants and an inspector will check the one you actually fitted.
3. Material declarations where relevant — RoHS and REACH for European destinations, food-contact declarations for food machinery.
4. Hazardous area certificates if any part of the machine is classified. See specifying intrinsically safe NAMUR loops.
5. Traceability records if your quality system or your customer requires component traceability to a batch.

CE marking and where the responsibility sits

Worth being precise, because it is widely misunderstood.

You CE mark the machine. The sensor manufacturer does not CE mark the machine, and a CE-marked sensor does not make your machine compliant. What component documentation does is support your technical file — it lets you demonstrate that the parts you selected are suitable for the use you put them to.

A sensor supplier who claims their CE mark covers your machine has misunderstood the framework. A supplier who provides clear documentation so you can build your own file is being useful.

Where sensor choice touches machine safety

If a sensor performs a safety function — guard interlock, safe position, emergency stop related — then the relevant standards are the machinery safety ones, and an ordinary proximity switch is generally not sufficient on its own.

Two things to get right regardless:

Fail-safe direction. Specify normally closed for guard and interlock positions, so that a cut cable, a disconnected plug or a failed sensor produces the same signal as an open guard. With normally open, a severed cable is indistinguishable from a closed guard, and the machine keeps running.

Diagnostic coverage. Where the control system must detect a failed sensor rather than merely respond to it, a conventional three-wire sensor cannot help — its off state and a broken wire look identical. NAMUR sensing provides genuine line fault detection, which is one reason it appears on machinery well outside hazardous areas.

Practical points for exporters

Voltage and frequency at destination. A machine going to a 110 V, 60 Hz market needs components rated for it. This catches out AC two-wire sensors in particular — see the A20C range.

Connector conventions. M12 connectors are effectively universal in European machine building. A hard-wired sensor where the customer expects a connector is a small irritation that reflects on the whole machine.

Spares availability at destination. Your customer will ask how they obtain a replacement. Being able to answer properly is part of the sale, and it is worth establishing before the machine ships rather than when the first one fails.

Marking legibility. Part numbers must still be readable after commissioning. A sensor whose label is destroyed by the first clean-down creates a support problem for the life of the machine.

Why domestic sourcing helps here

Three reasons specific to exporters.

Documentation comes from the manufacturer directly, in the timeframe you need it, rather than through an import chain that may not have it.

Variants are available. Destination-specific voltage, connector type and cable length can be specified rather than accepted, which removes adaptation work from every machine you ship.

The part stays available for the service life of the machine. An imported range that gets rationalised leaves your customer — and your reputation — without a replacement.

Accent manufactures in Mumbai to IEC 60947-5-2 and has done since 1985. Tell us the destination market and the documentation your customer requires, and we will confirm what comes with the delivery before you order rather than after.

A NAMUR proximity sensor cannot be wired to a PLC input. It has no output transistor and drives nothing. What it does is vary the current it draws from a nominal 8.2 V supply, and something has to provide that supply, interpret the current and turn it into a usable signal.

That something is the control unit, and treating it as an accessory rather than half the instrument is the most common mistake in NAMUR specification.

What the control unit does

Four jobs:

Supplies the sensor. A nominal 8.2 V, current limited.

Interprets the current. Above roughly 2.1 mA means undamped — no target. Below roughly 1.2 mA means damped — target present. The amplifier applies the threshold and produces a clean output.

Detects faults. Both normal states sit inside a narrow band, so anything outside it is a fault rather than a state. Near-zero current means a broken wire. Very high current means a short circuit. This is the property that makes NAMUR worth using even outside hazardous areas — a conventional three-wire sensor cannot distinguish between no target and cut cable, because both produce no output.

Provides isolation. In a hazardous-area installation the amplifier also limits the energy reaching the field circuit, which is what makes the loop intrinsically safe.

The Accent NAMUR control unit

Our NAMUR control unit converts the low-current sensor signal into a relay switching action, and works with either inductive or capacitive NAMUR sensors.

The architecture is the standard one: the sensor sits in the hazardous area and connects by two wires to the control unit, which is mounted in the safe area or inside a flameproof enclosure. That physical separation is the point — the only thing in the hazardous area is a two-wire device with no stored energy of consequence.

Three LEDs on the front fascia show power, cable fault and relay status. The cable fault indicator is the visible expression of the diagnostic band, and it is genuinely useful in service: a maintenance technician can see at a glance whether the problem is the sensor, the wiring or the logic, without a multimeter.

For non-hazardous applications where you want a switching amplifier without the intrinsic safety function, we also supply a standard proximity control unit. The full range sits under control units.

Relay output, and what it means for your design

A relay output gives you a volt-free contact, which has practical consequences worth planning for.

It suits any input. AC or DC, any voltage within the contact rating, PLC input or contactor coil. On retrofit work into older AC panels this matters, because there may be no DC logic anywhere.

It provides galvanic separation between field and control circuits.

It has a finite mechanical life. On a position interlock that operates a few times a shift, irrelevant. On something switching every few seconds, calculate the operations and check it against the contact life. If the count is high, a solid-state output is the better choice.

It has a switching time. Milliseconds rather than microseconds. Fine for process interlocks, not suitable for high-speed counting.

Specifying the loop rather than the parts

Six things to establish together rather than separately:

1. How many sensors, and therefore how many amplifier channels. Multi-channel units save DIN rail space where there are several.
2. Where the amplifier goes — safe area or flameproof enclosure — and what panel space is available.
3. What the output drives, with its voltage and current, so the contact rating can be confirmed.
4. Operations per hour, to check relay life against the duty.
5. Cable length and type, because intrinsic safety calculations depend on cable capacitance and inductance, and a long run changes the assessment.
6. Whether line fault detection must be wired through to the control system rather than only indicated locally. An LED nobody looks at is not a diagnostic; a fault contact routed to the PLC is.

That last point is worth dwelling on. The main practical advantage of NAMUR over conventional sensing is fault detection, and a great many installations throw it away by leaving the fault output unconnected. If you are paying for the diagnostic capability, wire it up.

The sensors

Our NAMUR inductive proximity switches cover M12, M18 and M30 with sensing from 4 mm to 15 mm: D20C-1204-NA, D20C-1805-NA, D20C-1808-NA, D20C-3010-NA and D20C-3015-NA.

Order sensor and amplifier together. Buying them from separate sources is how installations end up with a loop nobody can certify, and the saving never justifies it.

Hazardous area requirements

If the installation is in a classified area, the zone, gas group and temperature class come from your area classification document, and the approval scheme comes from the project specification. Our guide to specifying intrinsically safe NAMUR loops covers what to confirm, and the difference between ATEX, IECEx, UL and CSA.

Tell us the zone and the scheme when you enquire and we will confirm what we can supply against it, and what documentation comes with it.

Every few weeks we get an enquiry that reads roughly: we need an intrinsically safe NAMUR proximity switch, UL or CSA approved, suitable for Zone 0. It is a precise-sounding request that usually hides three separate decisions, and getting them confused is how projects end up with hardware that cannot be certified on site.

This guide separates them: what NAMUR actually is, what intrinsic safety requires beyond the sensor, and which approvals matter for which jurisdiction.

NAMUR is a signal standard, not a safety approval

This is the single most common misunderstanding. NAMUR — defined in IEC and EN 60947-5-6, and still occasionally called by its old designation DIN 19234 — specifies how a proximity switch communicates, not whether it is certified for a hazardous area.

A NAMUR sensor is a two-wire device with no output transistor. It does not switch a load. Instead it changes the current it draws from a nominal 8.2 V supply provided by an amplifier:

• Target absent (undamped) — the sensor draws a higher current, typically above roughly 2.1 mA.
• Target present (damped) — the sensor draws a lower current, typically below roughly 1.2 mA.

Because both normal states sit inside a defined, narrow band, anything outside that band is diagnostically meaningful. Near-zero current indicates a broken wire. Very high current indicates a short circuit. The amplifier can therefore distinguish a genuine fault from a legitimate sensor state — something a conventional three-wire PNP sensor simply cannot do, because its off state and a severed cable look identical.

That diagnostic capability is why NAMUR is used in process industries even outside hazardous areas. On a critical interlock, knowing that the sensor is healthy matters as much as knowing what it sees.

Why NAMUR and intrinsic safety go together

The second property of a NAMUR sensor is that it contains no energy storage of consequence and draws only milliamps. It is a simple apparatus in intrinsic safety terms. That makes it a natural building block for an intrinsically safe circuit, where the principle is to limit the electrical and thermal energy available in the hazardous area to below what can ignite the atmosphere present.

But the sensor alone does not create intrinsic safety. The loop does. An intrinsically safe installation is the sensor, the barrier or isolating amplifier, the cable, and the documented assessment that ties them together. Substituting any one element invalidates the case.

The barrier is not optional, and it is not an accessory

A NAMUR sensor cannot be wired directly to a PLC input. It needs an amplifier that supplies the 8.2 V, interprets the current, and presents a usable output to the control system. In a hazardous-area installation that amplifier also performs the energy limiting.

Our NAMUR control unit is built for exactly this arrangement: the sensor sits in the hazardous area and connects by two wires to the control unit, which is mounted in the safe area or inside a flameproof enclosure, and converts the low-current signal into a relay switching action. It works with inductive or capacitive NAMUR sensors, and carries three front-panel LEDs for power, cable fault and relay status — the cable fault indicator being the visible expression of the diagnostic band described above.

Specify the sensor and the amplifier together. Buying them separately from different sources is how loops end up unverifiable.

Zones, and why Zone 0 is a different conversation

Hazardous areas are classified by how often an explosive atmosphere is actually present:

• Zone 0 — present continuously, or for long periods. Inside tanks and vessels, typically.
• Zone 1 — likely to occur in normal operation.
• Zone 2 — unlikely in normal operation, and short-lived if it occurs.

Zone 0 is the demanding case, and it is where specifications need to be exact rather than approximately right. Equipment for Zone 0 requires the highest protection level, and the assessment covers the whole loop rather than the sensor in isolation. If your application is genuinely Zone 0 — inside a solvent tank, for example, rather than in the bunded area around it — say so at the enquiry stage. The difference changes the hardware.

It is worth confirming the zone classification with whoever produced your area classification drawing before ordering. A surprising number of enquiries specify Zone 0 for locations that are correctly classified Zone 1, which widens the available options considerably.

UL, CSA, ATEX, IECEx: which one you need

Approval schemes are regional, and the right one depends on where the plant is and who is signing it off.

• ATEX — the European scheme, and the one most often referenced in Indian project specifications because so much process plant is built to European standards.
• IECEx — the international scheme, widely accepted and often the most practical route for equipment crossing borders.
• UL and CSA — the North American schemes. These appear in Indian enquiries mainly where the end client, the licensor or the EPC contractor is American or Canadian, or where the plant is being built for export.

These are not interchangeable, and a certificate under one scheme does not automatically satisfy an inspector working to another. Establish which scheme your project actually requires before the technical bid evaluation, not after.

What to confirm before you order

1. The zone, gas group and temperature class from your area classification document — not an estimate.
2. Which approval scheme the project specification calls for, and who accepts it.
3. Whether the certification must cover the loop or only the sensor. This determines whether the barrier must be bought as a matched pair.
4. Sensing distance and target material, remembering that an inductive sensor sees non-ferrous metals at a reduced distance.
5. The thread size and sensing range your mounting allows.
6. Cable length and routing, because intrinsic safety calculations are affected by cable capacitance and inductance over long runs.

The Accent NAMUR range

We manufacture NAMUR inductive proximity switches in Mumbai in M12, M18 and M30 threads, with sensing distances from 4 mm to 15 mm:

• D20C-1204-NA — M12, 4 mm
• D20C-1805-NA — M18, 5 mm
• D20C-1808-NA — M18, 8 mm
• D20C-3010-NA — M30, 10 mm
• D20C-3015-NA — M30, 15 mm

Because approval requirements vary so much by project and by jurisdiction, tell us which scheme and which zone your specification calls for when you enquire, and we will confirm exactly what we can supply against it and what the documentation package looks like. That is a better conversation to have before the purchase order than during commissioning.

For background on how NAMUR sensors behave electrically, see our explainer on NAMUR proximity sensors.