Schneider Electric RM35HZ21FM RM35-HZ frequency relay

RM35HZ21FM: RM35HZ21FM frequency control relays in the Harmony range

The RM35HZ21FM is a Schneider Electric Harmony frequency control relays from the RM35 family, built to supervise overfrequency and underfrequency 50 or 60 Hz. It is powered from a separate 120–277 V AC supply, offers 1 changeover + 1 changeover output contact, and is housed in a 35 mm wide DIN-rail case. Seven Star LLC supplies the RM35HZ21FM in Oman and the GCC as an independent supplier based in Muscat.

The datasheet we reviewed is dated 8 Nov 2026. It describes a unit without a test button on the front.

Where the RM35HZ21FM is used in Oman and the GCC

Typical uses include standby and island generators, where the frequency tells you the engine is not at speed or the grid is unstable. It is also found in marine and offshore style networks and small isolated systems fed from engine-driven alternators, and in critical loads that should be disconnected or transferred when the supply drifts away from 50 or 60 Hz. Heat, dust, humidity and unstable supplies make supervision relays a low-cost way to protect larger equipment, but project specifications and local rules decide which protection is required.

Technical specifications of the RM35HZ21FM

Output contacts and durability of the RM35HZ21FM

The output is 1 changeover + 1 changeover contact, rated 5 A nominal at up to 250 V AC/DC, with a switching capacity of 1250 VA and a minimum switching current of 10 mA at 5 V DC. The listed durability is 100000 cycles electrical and 30000000 cycles mechanical. The contacts of the RM35HZ21FM are intended for pilot duty such as contactor coils, signal lamps and PLC inputs. For larger loads use an interposing contactor and keep the relay contact for the control circuit.

Because the minimum switching current is small, the same output can also feed a low-level signal such as a PLC input, but a contact that has switched heavy inductive loads should not later be reused for low-level signals.

Installing and commissioning the RM35HZ21FM

The relay clips onto a 35 mm DIN rail. The datasheet lists the operating position as Any position without derating. Terminals are screw type: Screw terminals, 1 × 0.5–1 × 4 mm² (20 AWG–11 AWG) solid without cable end; Screw terminals, 2 × 0.5–2 × 2.5 mm² (20 AWG–14 AWG) solid without cable end. Tightening torque is 0.6–1 N.m conforming to IEC 60947-1.

Before energising the RM35HZ21FM, check the nameplate range against the supply and compare the wiring with the diagram on the datasheet and on the side of the housing. Switch on and confirm the green supply LED and the output indication. Then provoke the monitored fault in a safe way, for example by removing a fuse on a spare circuit or by simulating the condition, and confirm that the output behaves as intended.

Settings, measurement and timing of the RM35HZ21FM

The measuring range is 40–70 Hz. Thresholds are set as -2–10 Hz or -10–2 Hz. Time delay: Adjustable 0.1–10 s, ± 10 % Tt- time delay upon fault. Hysteresis is 0.3 % fixed. Reset time is listed as 2000 ms time delay.

When commissioning the RM35HZ21FM, set the thresholds against the real running values of the plant, not against the nameplate alone. Where an adjustable fault delay is available, a longer time suits starting transients and short disturbances, while a short time protects equipment faster. Note the dial positions on the commissioning sheet so the settings can be restored after maintenance.

Environment, insulation and approvals for the RM35HZ21FM

Storage is listed at -40 to 70 °C. Operation is listed at -20 to 50 °C. Relative humidity is 95 % at 55 °C conforming to IEC 60068-2-30. Protection is IP20 (terminals) conforming to IEC 60529, pollution degree 3 conforming to IEC 60664-1, rated insulation voltage 400 V conforming to IEC 60664-1. The housing is polycarbonate.

Certifications listed on the datasheet for this reference: GL, UL, CSA, C-Tick, GOST. Check the marks against your project specification, because requirements differ between sites.

In a closed panel in an Omani summer the air near the relay can be much hotter than the room, so leave space around it and keep it away from contactors and power resistors.

What the RM35HZ21FM monitors

According to the datasheet, this relay covers the following functions. Each one is explained in practical terms below.

• Overfrequency and underfrequency. The relay measures the supply frequency and releases when it moves outside the window around 50 Hz or 60 Hz. Generator sets and weak islanded networks are typical places where frequency drifts before voltage does.

Information to send with an RFQ for the RM35HZ21FM

• Exact reference RM35HZ21FM and quantity.
• Supply voltage, frequency and type of network.
• The load or machine being supervised and the required alarm or trip behaviour.
• Required contact arrangement and rating.
• Related relays, sockets or accessories needed on the same order.
• Delivery location and timeline.

Documents and quotation

The Schneider Electric datasheet for the RM35HZ21FM is attached on this page and holds the complete specification summarised above, as printed on the manufacturer’s document dated 8 Nov 2026.

Send us the reference this unit, quantity and delivery location and Seven Star LLC will reply to your request. We supply as an independent supplier based in Muscat and do not act for the manufacturer.