What needs to be monitored?

Choosing a monitoring relay rarely starts with the type code. It starts with the question of which quantity is watched, and what should happen when it leaves the permitted range. The short version:

• Voltage too high, too low or gone – voltage monitoring, single- or three-phase. Often combined with phase sequence and phase failure in one device: a reversed rotating field runs a motor backwards, and on a pump that only shows when nothing is delivered.
• Current outside the window – current monitoring. Undercurrent is often the more telling signal: it shows the broken belt, the agitator running in an empty vessel, the tool that fell off.
• Insulation resistance dropping – insulation monitoring for unearthed systems. It reports the FIRST fault while the plant is still running; without it, the fault only shows with the second one, and then as a short circuit.
• Motor getting too warm – PTC thermistor tripping unit. It measures inside the motor rather than at the supply current, and therefore catches what a motor protection switch cannot see: blocked ventilation, too many starts, high ambient temperature.
• Shaft must stand still before anyone opens the door – standstill monitor, usually together with an interlock. A drive being switched off does not mean it has stopped.
• Something should switch delayed, pulsed or staggered – timer relay. Multifunction types cover a dozen functions and several time ranges, replacing a whole drawer of single-purpose devices.
• Evaluating emergency stop or a guard door – safety switchgear with positively driven contacts. Which design is required follows from the performance level demanded of the safety function.

You know the task but not the type? Tell us both – the quantity, the range and the reaction you want – and we will pick out the right item.