FM SeriesHeavy-Duty Paddle Flow Switch

Built tough for the most arduous industrial and mining applications. Rugged but highly-sensitive paddle switch with a billet-aluminium housing and a one-piece 316 stainless steel process connection. 400 bar pressure rating, manual override built into all models.

Features

5–500 V AC, 15 A S.P.D.T. switch (H model)
Low-voltage gold-contact 'L' model available (FM-L)
One-piece 316 stainless steel process connection
Super-tough billet aluminium housing
Manual override built in
3/4" and 1" models available with BSP or NPT threads
400 bar (5800 psi) pressure rating
Seal-less magnetic drive
Dieseline & high-temperature versions available
Weatherproof IP67 housing
Easily serviceable

The FM Series uses the same magnetic-coupling drive as the F Series — paddle motion is coupled to the high-compliance switch through a solid wall of stainless steel and aluminium, giving a high-pressure switch with no seals, diaphragms, bellows or other points of potential failure. Supplied as standard with a one-piece 316 stainless steel process connection and a fully machined billet aluminium housing. Available with a glass-fibre-reinforced polypropylene paddle, or a nickel copper alloy paddle for Dieseline and high-temperature applications. The paddle can be cut and shaped to suit pipework 25 mm (1") or larger, and the built-in manual override switches the flow switch on at the press of a button regardless of flow. Suitable for use in bore water, sea water and many chemical solutions.

More Information

Ordering — Available Models

Model Number	Description
FM-H-25B	Standard heavy-duty switch (H) — 1" BSP thread, polypropylene paddle for water use
FM-H-25B-D	Standard heavy-duty switch (H) — 1" BSP thread, nickel copper alloy paddle for Dieseline and high-temperature applications
FM-H-25N	Standard heavy-duty switch (H) — 1" NPT thread, polypropylene paddle for water use
FM-H-25N-D	Standard heavy-duty switch (H) — 1" NPT thread, nickel copper alloy paddle for Dieseline and high-temperature applications
FM-H-20B	Standard heavy-duty switch (H) — 3/4" BSP thread, polypropylene paddle for water use
FM-H-20B-D	Standard heavy-duty switch (H) — 3/4" BSP thread, nickel copper alloy paddle for Dieseline and high-temperature applications
FM-H-20N	Standard heavy-duty switch (H) — 3/4" NPT thread, polypropylene paddle for water use
FM-H-20N-D	Standard heavy-duty switch (H) — 3/4" NPT thread, nickel copper alloy paddle for Dieseline and high-temperature applications
FM-L-25B	Low-voltage gold-contact model (L) — 1" BSP thread, polypropylene paddle for water use
FM-L-25B-D	Low-voltage gold-contact model (L) — 1" BSP thread, nickel copper alloy paddle for Dieseline and high-temperature applications
FM-L-25N	Low-voltage gold-contact model (L) — 1" NPT thread, polypropylene paddle for water use
FM-L-25N-D	Low-voltage gold-contact model (L) — 1" NPT thread, nickel copper alloy paddle for Dieseline and high-temperature applications
FM-L-20B	Low-voltage gold-contact model (L) — 3/4" BSP thread, polypropylene paddle for water use
FM-L-20B-D	Low-voltage gold-contact model (L) — 3/4" BSP thread, nickel copper alloy paddle for Dieseline and high-temperature applications
FM-L-20N	Low-voltage gold-contact model (L) — 3/4" NPT thread, polypropylene paddle for water use
FM-L-20N-D	Low-voltage gold-contact model (L) — 3/4" NPT thread, nickel copper alloy paddle for Dieseline and high-temperature applications

Operating Limitations

Maximum operating pressure (FM with polypropylene paddle, ambient)	400 bar / 5800 psi
Maximum operating pressure (FM-D with metal paddle, ambient)	400 bar / 5800 psi
Minimum burst pressure (both models)	800 bar / 11 600 psi
Maximum operating temperature (polypropylene paddle)	80 °C / 176 °F
Maximum operating temperature (metal paddle, FM-D)	100 °C / 212 °F
Minimum operating temperature (both models)	-60 °C / -76 °F
Ingress protection rating	IP67

Electrical Data

Switch type	S.P.D.T. (single pole, double throw) standard heavy-duty microswitch for general control circuits up to 500 V AC — ideal for pump starters, relay logic, contactors and timers
Voltage range	5 to 500 V AC (minimum load 160 mA at 5 V DC)
Maximum switched current	15 A (10 A at 500 V AC). Maximum voltage and maximum current cannot be applied together.
Resistive or inductive load (125 V AC or 250 V AC)	15 A
Resistive / inductive load at other voltages	500 V AC: 10 A / 6 A; 8 V DC: 15 A / 15 A; 14 V DC: 15 A / 10 A; 30 V DC: 6 A / 5 A; 125 V DC: 0.5 A / 0.05 A; 250 V DC: 0.25 A / 0.03 A
Lamp load (NC / NO)	3 A / 1.5 A at 125 V AC; 2.5 A / 1.25 A at 250 V AC; 1.5 A / 0.75 A at 500 V AC; 3 A / 1.5 A at 8, 14 and 30 V DC; 0.5 A / 0.5 A at 125 V DC; 0.25 A / 0.25 A at 250 V DC
Motor load (NC / NO)	5 A / 2.5 A at 125 V AC; 3 A / 1.5 A at 250 V AC; 1.5 A / 0.75 A at 500 V AC; 5 A / 2.5 A at 8, 14 and 30 V DC; 0.05 A / 0.05 A at 125 V DC; 0.03 A / 0.03 A at 250 V DC
Direct motor control	Directly switches pump motors up to 1/4 HP (approx. 185 W) at 240 V AC. For larger motors use a contactor or relay.
FM-L switch type	Gold contact microswitch, S.P.D.T., gold alloy contacts, so it switches reliably at the low wetting currents of PLC inputs, telemetry and relay logic. Not suitable for inductive or capacitive loads such as contactors and relays. Loads above the rating will damage the switch.
FM-L rated current (resistive)	0.1 A at 125 VAC; 0.1 A at 8, 14 and 30 VDC
FM-L inrush current	0.1 A maximum (NC and NO)
FM-L minimum applicable load	1 mA at 5 VDC
FM-L operating range (micro load)	5 to 30 V, 5 to 800 mW (26 mA maximum at 30 V)
FM-L contact resistance	50 mΩ maximum (initial)
FM-L typical applications	PLC, telemetry and low voltage control

Dimensions

Manual Override

All FM models include a red manual override button on the side of the housing, under the cable gland. Pressing it switches the flow switch on regardless of flow — useful for manually starting pumps in spite of an initial lack of flow, and for testing/commissioning of control circuits. After wiring, press it to test: the switch changes state as if liquid were pushing the paddle.

Flow Sensitivity & Paddle Trimming

Cut the paddle to suit the application — it must clear the pipe wall by at least 4 mm all round. Use a hacksaw or tin snips (heavy-duty tin snips for the FM-D metal paddle). As a guide: low flow — paddle just clears the bottom of the pipe; unknown flow — 3/4 of the way across the pipe; high flow — 1/4 to 1/2 of the way across the pipe. Spare paddles are available if too much is cut off. The flow rates required to actuate depend on turbulence, viscosity, and the exposed paddle area. Use the online Paddle Calculator to estimate activation flow and determine the right cut.

Open the Paddle Calculator →

Sensitivity Adjustment

FM switches are supplied at maximum sensitivity (adjuster wound fully in). To decrease sensitivity, remove the red dust cap and wind the adjusting screw out (anticlockwise) with a wide flat screwdriver. The further out the screw, the more flow is needed to operate the switch. Refit the dust cap.

Dieseline / High-Temperature Variant

Order the -D suffix (e.g. FM-H-25B-D) for dieseline service or operation up to 100 °C. The polypropylene paddle is replaced with a Nickel Copper alloy paddle. Kelco products are not intended for use in hydrocarbons other than dieseline.

Mounting

The switch can go in suction or delivery pipework, horizontal or vertical. In vertical pipework the flow must be upward, not downward. Never install the switch on the underside of horizontal pipework. Where possible, mount the switch away from pumps, valves and bends — a fitting directly upstream makes the flow uneven and turbulent at the paddle, which can shift the switching point slightly or make the paddle flutter near the switching point; downstream fittings have little effect. Mount in a pipe tee, tapping saddle or weld-on socket, square to the flow so liquid hits the flat face of the paddle, with the arrow on the switch pointing in the direction of flow. Seal the thread with PTFE tape or thread sealant, leaving at least 4 mm of thread above the socket, and tighten with a spanner on the spanner flats only — never twist the switch body.

Electrical Installation

The FM Series is a mains voltage device — all electrical work must be carried out by a licensed electrician. Terminals are marked C (common), NO (normally open) and NC (normally closed); for a two-wire connection use C plus either NO or NC to suit the application. Connect the earth terminal inside the housing: the FM must be earthed when operated at mains voltage. After wiring, fully tighten the cable gland nut, check the lid gasket is in place and tighten the lid down. Without its lid the switch is not water, weather or insect proof and presents a shock hazard.

Hazardous Areas

The FM-H is not certified for use in hazardous areas and must not be installed in any area classified as hazardous under AS/NZS 60079.10 (Zone 0, 1, 2, 20, 21 or 22). The FM-L is simple apparatus under AS/NZS 60079.11 and may be used in a hazardous area only as part of an intrinsically safe circuit, connected through suitably certified associated apparatus. It is not Ex certified in its own right. The associated apparatus must not exceed Uo ≤ 30 V, Io ≤ 100 mA. The full conditions of use are given in the installation sheet.

FM-L — Inductive and Capacitive Loads NOT Suitable

The FM-L is not suitable for inductive or capacitive loads such as contactors and relays. Loads above the rating will damage the switch.

Replacing the Paddle

The paddle and its associated pivot bearings are replaceable items. Spare paddles can be obtained from your supplier — please quote the part number of the flow switch when ordering. Replacement paddles are supplied with new bearing shells and a large retaining O-ring. On FM-D models the nickel copper alloy paddle, main arm and bronze pivot bearing are also replaceable; never undo or remove the sealed hexagonal cap at the top of the main arm, which houses the magnet.

Materials & Chemical Compatibility

All FM models have a one-piece 316 stainless steel process connection and a billet aluminium housing. The standard FM has a glass-fibre-reinforced polypropylene paddle for water use; the FM-D has a nickel copper alloy paddle. Suitable for use in bore water, sea water and many chemical solutions. Check that the switch materials suit the liquid being monitored.

Approved Standards

The 'H' and 'L' microswitches used in the FM Series are approved to UL1054 (File E41515), CSA C22.2 No.55 (File LR21642) and EN61058-1, and are CE marked.

Serviceability

Fully field-serviceable. The paddle and its pivot bearings are replaceable items (on the FM-D, the nickel copper alloy paddle, main arm and bronze pivot bearing). The electrical housing is a separate assembly that locks onto the wet-end with a single externally-accessible screw — swap modules without disturbing pipework. Replacement paddles, seals and switches available from Kelco Engineering or your distributor.