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:
- How many sensors, and therefore how many amplifier channels. Multi-channel units save DIN rail space where there are several.
- Where the amplifier goes — safe area or flameproof enclosure — and what panel space is available.
- What the output drives, with its voltage and current, so the contact rating can be confirmed.
- Operations per hour, to check relay life against the duty.
- Cable length and type, because intrinsic safety calculations depend on cable capacitance and inductance, and a long run changes the assessment.
- 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.
Need a price, a drawing or a stock check?
Send us the part number, or just describe the application and the target you need to sense. We will come back with the right model, a dimensional drawing and a quotation. Accent Controls has built sensors in Mumbai since 1985 and is the sole authorised distributor in India and SAARC for Novotechnik (Germany) and Contelec (Switzerland).
Request a quotation · +91 98673 64004 · info@accentsensors.com