A bearing-temperature alarm loop fails audits for the same reason it fails in service: the certificate, the sensor, the thermowell, and the setpoint were specified by four different people on four different days. A Pt100 RTD probe on a motor or pump bearing housing is the most common machine-protection temperature channel on a power-plant or chemical-plant asset, and the certification chain that backs it is what an inspector, a reliability engineer, or an insurance surveyor will read first when something trips [S1].
The minimum defensible spec is a Pt100 Class A element (IEC 60751) on a 3-wire or 4-wire lead run, terminated in an IP65 or higher connection head, installed through a stainless-steel thermowell, with an alarm at 80°C and a trip at 95°C at the bearing housing, and a hazardous-area certificate that matches the classified zone around the motor terminal box or pump frame [S1][S2]. Each of those items has a specific evidence trail; this checklist walks through them in the order an auditor will read them.
Sensor Element: Pt100 Class and Wiring
Pt100 is the default element for bearing-temperature work because the platinum curve is stable, the interchangeability is defined, and most plant safety PLCs accept the 100 Ω at 0°C input directly [S3]. For bearing housings, the working temperature sits well inside the element's useful span, so the choice is really about accuracy class and lead resistance, not upper range.
Specifying Class A (IEC 60751) gives ±(0.15 + 0.002|t|) °C, which is tighter than the older Class B ±(0.30 + 0.005|t|) °C tolerance and removes a meaningful slice of measurement uncertainty from a 4–6 K alarm window. Use a 3-wire circuit as the practical minimum on a motor or pump housing; use a 4-wire circuit where the lead run exceeds roughly 10 m, where the conduit shares runs with VFD output cable, or where the spec demands ratiometric cancellation of lead resistance. 2-wire builds are acceptable only for short pigtails into a head-mounted transmitter [S1].
Probe, Insert, and Thermowell Build
A bearing-housing probe is not a bare element; it is an assembly, and the auditor will look at it that way. The standard build is a MgO mineral-insulated stainless-steel sheath (typically 316L or Inconel 600 for higher-temperature service), 3 mm or 6 mm OD, terminated in a spring-loaded insert so the tip stays in contact with the thermowell base, with the head and lead exit selected to suit the cable run [S1].
The thermowell is what actually sees the process. Specify a stainless body (316/316L is the default; upgrade for corrosive or coastal service), a process thread that matches the bearing-housing boss (typically 1/2" NPT or G1/2), and a lagging extension long enough to clear any insulation on the housing. The thermowell is the part that gets pulled for ASME B31.3 or plant piping inspections, and it should be ordered with a material certificate (EN 10204 3.1) so the weld procedure and alloy are traceable. A common audit finding is a probe installed without a thermowell directly into an oil drain, which works in service but is not repairable and fails the inspector's "is this a maintainable instrument loop?" question [S1].
Connection Head, Transmitter, and Wiring
The connection head protects the terminals, holds the transmitter if one is fitted, and carries the conduit entries and the Ex marking. For a plant-floor bearing housing, an IP65 die-cast aluminum head is the typical minimum; specify stainless or polyester-coated heads in corrosive or wash-down areas. Conduit entries should be selected for the cable type: NPT for North American plants, metric for IEC 60079-10 installations, with cable glands rated to the same Ex concept as the head [S1].
Two wiring paths are common. A direct-wire path lands the RTD leads on a terminal block and runs copper or nickel-shielded extension back to the PLC or RTU analog input. A transmitter path puts a 4-20 mA head-mounted transmitter in the head and runs a single twisted pair to the controller, which is preferred when the lead run is long, when the controller's RTD input channel is scarce, or when the same physical loop has to coexist with VFD cable. The transmitter must share the same Ex certificate family as the head if the area is classified; an Ex d head with an Ex i transmitter inside it is an installer error that an Ex inspector will catch.
Hazardous-Area Certification: ATEX, IECEx, and the Zone Map
Most motor and pump rooms in oil & gas, chemical, and petrochemical plants sit in Zone 1 or Zone 2, and the RTD assembly is a piece of equipment that goes inside that boundary. The certification chain is the part of the spec where errors cost the most: a probe with a perfectly good Pt100 element and a perfectly good thermowell, installed under a head marked for the wrong zone, turns the whole loop into a non-compliant device. The four marks that come up in procurement documents are ATEX (2014/34/EU) for the EU, IECEx for IEC 60079 member countries, UL/CSA Class/Division for North American plants, and regional schemes (NEPSI, INMETRO, KCS) for local acceptance [S1].
Read the certificate before you read the datasheet. Each certificate has a "Equipment Protection Level" or "EPL" tied to a specific zone (Ga/Da for Zone 0, Gb/Db for Zone 1, Gc/Dc for Zone 2), a specific gas group (IIA, IIB, IIC) and a temperature class (T1–T6). For a motor terminal box or pump frame, the typical requirement is Ex db IIC T4 or T5 (flameproof) or Ex eb IIC (increased safety) with EPL Gb for Zone 1, or EPL Gc for Zone 2. A common audit finding is a sensor marked "Ex d" on the head but installed with a "non-sparking" or ordinary cable gland; the gland and the head have to be on the same certificate chain, and the installer has to follow the certificate's "Conditions of Use" clause, which usually restricts cable entries to specific torque values and thread types. The certificate number, the issuing notified body, and the "Ex" marking string should all appear on the head nameplate and on the shipping documents; verify them against the issuing body's public database rather than against the manufacturer's PDF [S1].
Setpoints, Alarms, and Trip Logic
The certification chain does not stop at the head; it runs through to the trip. For oil-lubricated bearings on a circulating-water pump, the published industry reference is an alarm at 80°C and a trip at 95°C at the bearing housing, with the alarm trigger widened to "40°C above ambient" when the installation is in a cold climate, because the absolute number alone produces nuisance trips in winter [S2]. The matching vibration envelope is roughly 4.5 mm/s RMS on a rolling-element bearing, and the two channels are read together: a bearing that is hot but quiet is a lubrication problem, a bearing that is hot and vibrating is a mechanical problem, and the alarm logic should reflect that.
Setpoint selection has to be done with the bearing OEM's data sheet in hand, not from a generic table. The 80°C/95°C numbers are a useful cross-check, but a heavy-duty spherical-roller bearing on a slow-speed mill may have a higher continuous rating, and a high-speed motor bearing may have a lower one. Document the basis: bearing type, calculated L10 life, lubricant grade, ambient, and the OEM's published temperature limit, then derive the alarm and trip from that basis with the industry numbers as a sanity check.
Documentation and Audit Trail
An RTD assembly is only as good as the paperwork in the maintenance system. For each loop, the inspector will want to see: the sensor calibration certificate (often a 0°C and 100°C shop reading traceable to a national lab), the thermowell material certificate (EN 10204 3.1), the head/transmitter Ex certificate with the conditions of use, the manufacturer's IEC 60751 conformity statement, the loop diagram, the installation drawing, and the commissioning record showing insulation resistance, loop resistance, and a known-temperature check [S1].
For the broader asset context, a PT100 RTD primer covers the resistance-vs-temperature curve and tolerance classes, the safety certification reference explains how to read an ATEX/IECEx certificate string, and a temperature controller selection page documents the input-card side of the same loop.
Until those settle, the safe procurement path is to keep the spec conservative: Pt100 Class A, 3- or 4-wire, MI sheath, 316L thermowell, IP65 head, Ex db IIC T4/T5, with the 80°C alarm and 95°C trip written into the loop logic and the certificate numbers copied into the maintenance system before the pump starts.