Engineering notes

Every manufacturer codes its sensors differently, but the same six facts are in there somewhere, and they are the six that decide whether a replacement will work. Take the designation apart and you have your shopping list.

What must match

• Housing size and thread. M8, M12, M18 or M30: the thread the bracket is drilled for. A different size means new brackets and a new sensing distance.
• Flush or non-flush. A flush (shielded) sensor can be mounted level with a metal bracket; a non-flush one needs free space around its face but sees roughly twice as far. Swap one for the other and it will either false-trigger on the bracket or not reach the target.
• Sensing distance, Sn. The nominal switching distance to a standard steel target. The replacement should be equal or greater, and remember that aluminium, brass and stainless reduce it (the datasheet gives a correction factor).
• Output type. PNP (sourcing) switches the load towards 0 V; NPN (sinking) switches it towards +24 V. Most PLC input cards sold in the UK and Europe are wired for PNP. Get this wrong and the input never sees the signal.
• Normally open or normally closed, or both (antivalent, 4-wire). Match what the program expects.
• Connection. An M12 or M8 connector, or a fixed cable of a given length and jacket (PUR for oil and drag chains, PVC for general use). A connector version plus a cordset is usually the easiest spare to hold.

Worth checking too

Supply voltage (10–30 V DC covers almost everything), the IP rating where it's fitted (IP67 is standard; washdown wants IP68/IP69K and a stainless housing), switching frequency on fast machines, and whether the original had a correction factor of 1 (the same distance for all metals) that the application may rely on.

Typical sensing distances, for orientation only: M8 about 1.5–2 mm flush and 4 mm non-flush; M12 2–4 and 4–8 mm; M18 5–8 and 8–12 mm; M30 10–15 and 15–22 mm. Always confirm against the datasheet.

A 24 V DC supply looks like the simplest part in the panel, which is why replacements go wrong. Here is what to compare, in the order it matters.

Must match

• Output voltage. 24 V DC in most panels; also 12 V and 48 V. Most units are adjustable (typically 24–28 V) so a long cable run can be compensated. Note what the old one was set to before you remove it.
• Output current. Equal or greater than the old unit. If the old one was running hot, measure the real load with a clamp meter rather than copying the rating.
• Input. Single-phase 100–240 V AC, or three-phase 400–500 V AC. They are not interchangeable, and a three-phase unit will not run from one phase without losing most of its rating.
• Size and terminals. Width in millimetres on the rail, depth under the door, and whether neighbouring parts leave room for the ventilation gap the datasheet specifies (often 50 mm above and below).

Decides whether it behaves the same

• Derating. Most units give full power up to about 60 °C and lose roughly 2.5% per degree above that. A panel in a warm plant room can quietly turn a 10 A supply into a 7 A one.
• Overload behaviour. Some units deliver a short boost (150% for a few seconds) to start motors and charge capacitors; others go into hiccup mode and restart. If the old unit had boost and the new one doesn't, loads that always started may now not.
• Tripping breakers. A current-limited supply often cannot deliver enough fault current to trip a downstream MCB quickly, so a short on one branch pulls the whole 24 V rail down. The usual answers are a supply with a defined boost for selective tripping, or electronic circuit protection on each branch.
• Redundancy. Two supplies in parallel need decoupling, either a redundancy module or units with built-in decoupling diodes, and both should be set to the same voltage. Paralleling two ordinary units without it shares current badly and can damage one of them.
• Signalling. A DC-OK contact or LED lets the PLC know the supply is healthy. If the old unit had one wired, the new one needs it too.

Hold-up time (how long the output rides through a mains dip, typically 20–30 ms at full load) matters for lines that must not reset on a flicker; buffer modules extend it to seconds.

An interface relay sits between a PLC output and a load the output can't drive directly, or isolates one circuit from another. A 24 V DC coil, one or two changeover contacts rated around 6 A, often a slim plug-in module on a base so the relay can be swapped without rewiring. Choose by coil voltage, contact arrangement and rating, base width, and whether you want an LED and a protection diode.

A safety relay is a different animal. Its contacts are force-guided (mechanically linked), so a welded contact is detectable rather than silent. It monitors two input channels and detects a short between them. It has a feedback loop that checks the contactors it controls have actually dropped out before it allows a restart. And it has defined start modes: automatic, or monitored manual reset so the machine can't restart just because the guard was closed.

When a safety relay is called for

Wherever the machine's risk assessment assigns a required performance level (PLr) to a function such as an emergency stop, a guard interlock or a light curtain, under EN ISO 13849-1 (or a SIL under IEC 62061). The performance level achieved depends on the whole chain, the sensor, the logic and the contactors it switches, so swapping a safety relay for an interface relay, or for a safety relay of a lower category, changes the safety of the machine even if everything appears to work.

Replacing one

• Match the category and performance level the circuit was designed to, and the number of safety and auxiliary contacts.
• Match the input configuration: one or two channel, contact or OSSD (solid-state, as from a light curtain), and the reset mode.
• Keep the feedback loop wired. Bridging it out to make a stubborn machine start is a common and dangerous shortcut.
• Make sure whoever signs off the change is competent to do so. We supply the part and its datasheet; the safety design and validation belong with your engineer or integrator.

When a part is current, the quote simply says new. When it isn't, the words matter, because they tell you where the part has been and what stands behind it.

New

Current production from the manufacturer's distribution. Full manufacturer's warranty, recent date code, original packaging and documents.

New, unused old stock

Never fitted, usually in original packaging, but the part is out of production or the stock has sat for years. Electrolytic capacitors and batteries age on the shelf, so for power supplies and drives we tell you the date code where we can. The warranty is the one stated on the quote, which may be the seller's rather than the manufacturer's.

Surplus

New and unused, from a cancelled project, an over-order or a plant that closed. Packaging may be missing or opened. Tested before dispatch where we can arrange it; the quote says so.

Refurbished

Previously fitted, then cleaned, tested and repaired by a specialist, with worn parts replaced. The quote states who refurbished it and the warranty they give. For a drive or a PLC CPU this is often the only realistic way to keep an older machine running.

Three habits that save trouble

• Ask for the date code on anything with capacitors in it that is more than a few years old.
• For a PLC CPU or HMI, check firmware and whether your program and its licence will load on the replacement unit before it arrives, not after.
• Keep the old part until the new one has run for a week. If the fault was elsewhere, you still have a known-good spare.