Undervoltage, coordination and monitoring — 535 to 538Why should only the nearest device trip?

These are short sections. You don't learn them, you locate them — recognise the keyword in the question and flick straight there.

535 — protection against undervoltage

This is the device-selection end of 445, which you met in Part 4 earlier. The danger is what a drop or loss of voltage, and its return, can do: equipment restarting by itself when the voltage comes back, with someone's hands on it, or equipment being damaged by running on too low a voltage. The regulation is mostly framed around relays and motor circuits, but the principle applies to any circuit.

536 — coordination, also known as selectivity

Section 536 is headed coordination. Selectivity is the same idea under its other name, and it is defined in Part 2 (p.48). Know both words.

• Selectivity applies to overload conditions as well as short circuits and residual currents.
• You can have selectivity between fuses and breakers, and between overcurrent devices and RCDs.
• There is more than one way of achieving it, and the section lists options. 536.3 lists the requirements — that's the bit worth highlighting.

Don't confuse it with the problem from earlier: put too many circuits behind one 30 mA RCD and accumulated leakage trips it with no fault present. That isn't selectivity; it's a design and division-of-circuits problem (531.3.2: standing leakage no more than 30 % of I_Δn).

538 — monitoring devices

This is where insulation monitoring devices (IMDs) for IT systems sit, alongside residual current monitoring. Don't muddle an IMD with an RCM — different job, different device.

535 · devices for protection against undervoltage 536 · coordination / selectivity ★ 536.3 · the list of requirements 538 · monitoring devices · IMDs for IT systems 445 · undervoltage (Part 4)