Shielded vs Unshielded Inductive Sensors: Flush Mounting, Spacing and Metal-Free Zones

A shielded (flush) inductive sensor has a metal band around its coil that confines the field to the front face, so it can be buried level with a steel bracket. An unshielded (non-flush) sensor lets the field spread sideways, giving 1.5–2× the sensing range but demanding a metal-free zone around the head.

What the shield actually does

Both types generate the field the same way — an oscillator drives a coil wound on a ferrite pot core. The difference is what surrounds that core.

In a shielded sensor, a copper or steel band wraps the circumference of the coil assembly. It absorbs the lateral component of the field and forces the flux to emerge only from the front. The result is a field shaped roughly like a short cylinder projecting from the face. Because energy that would have gone sideways is now dissipated in the shield, the forward reach is shorter — but the sensor is blind to metal beside it, so it can be threaded into a steel bracket right up to the face.

In an unshielded sensor, the core is open. The field balloons outward and forward, reaching further along the axis but also extending well past the barrel diameter. Surround that head with steel and the sensor sees the mounting, not the target, and latches permanently on.

Range comparison by thread size

Typical rated operating distances (S_n) for standard-range inductive sensors detecting mild steel:

Thread size	Shielded Sn	Unshielded Sn	Gain
M8	1.5 mm	2.5 mm	1.7×
M12	2 mm	4 mm	2.0×
M18	5 mm	8 mm	1.6×
M30	10 mm	15 mm	1.5×

Remember these are S_n figures. The distance you can actually design to is the assured operating distance, 0.81 S_n, further multiplied by the reduction factor for your target material. An unshielded M12 on aluminium gives 4 × 0.81 × 0.4 ≈ 1.3 mm, not 4 mm.

The metal-free zone, precisely

This is where most unshielded installations go wrong. Three separate clearances have to be satisfied:

1. Radial clearance around the head

Clear a circle of at least three times the sensor’s barrel diameter, centred on the sensing face. For an M18 unshielded sensor that means a 54 mm clear diameter — a 20 mm hole in a steel plate is nowhere near enough.

2. Axial protrusion from the mounting

The head must stand proud of surrounding metal by at least twice the rated operating distance. An unshielded M18 at S_n = 8 mm must project 16 mm beyond the bracket face.

3. Clearance to opposing metal

Any metal surface facing the sensor that is not the target must be at least three times S_n away. Machine guards, chute walls and tramp steel in the background all count.

If your mechanical design cannot give you all three, you do not have an unshielded application. Fit a shielded sensor and accept the shorter range, or move to a larger thread size.

Mutual interference between sensors

Two inductive sensors mounted close together can beat against each other. Their oscillators run at similar frequencies, the fields couple, and the result is an audible-frequency modulation that shows up as random chattering on both outputs. It is intermittent, it worsens as the units warm up, and it is very hard to diagnose if you do not know to look for it — it appears in our list of common proximity sensor faults for exactly that reason.

Arrangement	Shielded minimum	Unshielded minimum
Side by side (centre to centre)	2 × barrel diameter	3 × barrel diameter
Facing each other	6 × Sn	12 × Sn
Opposed across a gap, same axis	Avoid — use a mechanical barrier or one sensor	Avoid

Where the mechanical layout genuinely forces sensors closer than this, specify units with different oscillator frequencies, or use a design intended for tight arrays. Wiring them in series or parallel does not solve interference — that is a load and voltage-drop question, covered in our note on series and parallel connection of proximity switches.

Which to choose

Situation	Choose	Why
Sensor threaded into a steel machine frame	Shielded	Only type that can sit flush without latching on
Detecting through a narrow slot or between fixtures	Shielded	Focused field ignores the surrounding fixture
End-of-stroke on a hydraulic cylinder	Shielded	Cylinder body is steel and close on all sides
Long gap to the target, open mounting	Unshielded	Range gain of 1.5–2× for the same thread size
Detecting non-ferrous targets at distance	Unshielded	Recovers range lost to the reduction factor
Sensor mounted on a plastic or aluminium bracket, clear space around	Unshielded	Metal-free zone is satisfied naturally
High-density sensor array	Shielded	Tighter spacing permitted, less mutual interference

Semi-flush and other variants

Some ranges offer a semi-flush or “quasi-flush” option: partially shielded, mountable with a small recess, giving a range between the two extremes. It is a useful compromise when the drawing has already been cut and the metal-free zone is almost big enough. Treat the manufacturer’s stated mounting condition as binding — a semi-flush unit installed truly flush behaves like an unshielded one and will latch.

Once you have chosen the type, the mechanical installation still matters: tightening torque, locknut position and cable strain relief all affect long-term reliability. Those are covered in our guide to installing a proximity switch correctly.

Frequently asked questions

What happens if I mount an unshielded sensor flush in metal?

It detects the mounting bracket and its output latches permanently on, regardless of the target. The sensor is not damaged and will work normally once it is moved proud of the metal by at least twice its rated operating distance. This is the single most common commissioning fault with unshielded units.

Can a shielded sensor be mounted with the face recessed below the metal?

Slightly, but every millimetre of recess is subtracted from your working gap, and the surrounding metal begins to load the field once the recess approaches the sensing distance. Mount flush or very slightly proud. If the design needs a genuine recess, size the sensor for the recess depth plus the target gap.

Is an unshielded sensor always the better choice when I need more range?

Only if the metal-free zone can be met. If it cannot, a larger shielded sensor is the correct answer — an M30 shielded unit at 10 mm beats an M18 unshielded unit at 8 mm and mounts in metal. Compare the assured distance after material correction, not the headline figures.

Do these rules apply to capacitive sensors too?

The flush and non-flush distinction exists for capacitive sensors as well, and the mounting logic is the same. The clearances differ, and capacitive units are far more sensitive to build-up on the face and to surrounding dielectrics such as plastic guards, not just metal.

How do I know if my sensor is shielded or unshielded?

The datasheet states it directly, usually as “flush mountable” or “embeddable” versus “non-flush” or “non-embeddable”. Physically, unshielded sensors often have a sensing face that protrudes slightly as a plastic cap wider than the thread root, and their rated range is noticeably longer for the same thread size.