Emergency Generators & Low-Voltage Generation — §551
§551 — Emergency Generators & Low-Voltage Generation
§551 (IEC 60364-5-55, clause 551 — "Low-voltage generating sets") governs low-voltage generating installations: emergency generators, safety supplies, and other local generation sources that can be deployed alongside or instead of the public mains.
Note: this chapter covers the core principles of §551 based on independently verified sources. Specific numbered sub-clauses (for example exact sub-clause numbers within §551) cannot be verified from primary, freely accessible sources — for a concrete design, always consult the full NEN 1010 text itself for the exact sub-clause.
Mutual exclusion during switching
The core principle of §551: the switching device between the public mains and the generator must be designed so that both sources mutually exclude each other — never simultaneously, unsynchronised, connected to each other. The reason is physically concrete: two unsynchronised sources that accidentally end up connected in parallel can cause a fault current that is higher than the prospective short-circuit current for which the installation and its protective devices are dimensioned — with a real risk of damage to the installation and the generator.
In practice this is realised with a changeover switch (manual or automatic, for example an ATS — automatic transfer switch) that mechanically or electrically guarantees that both sources are never connected to the installation at the same time.
Star-point earthing during parallel operation
When a generator can run in parallel with the public mains for longer than briefly (rather than only as an emergency supply during an outage), the star-point earthing of the generator must be assessed separately. A generator that only serves as a replacement for the mains (never connected simultaneously) needs a different earthing assessment than a generator that structurally runs in parallel with the mains — the assumptions behind the fault current calculation and the protective coordination change when there are two simultaneously active sources, each with its own earthing point.
Practical applications
- Emergency power supplies for buildings with a safety function (escape-route lighting, lifts, medical rooms) — the generator kicks in during a mains outage, never in parallel.
- Construction sites or event venues where a generator is the only source (no mains connection present) — here the switching requirement does not apply, but the regular §704 requirements do if it also concerns a construction site.
- Businesses with their own generation that deliberately run in parallel with the mains for load sharing — here the star-point earthing assessment is essential, not optional.
Common mistakes
- Using a changeover switch that does not guarantee mechanical/electrical mutual exclusion — the risk of unsynchronised parallel switching of mains and generator then remains.
- Not assessing the generator's star-point earthing separately for (intended) parallel operation — this assessment differs fundamentally from the situation where the generator only serves as a replacement.
- Assuming every emergency generator needs the same protection regime — a generator that never runs in parallel has different requirements than a generator that structurally runs alongside the mains.
- Confusing §551 with §712 (PV) — both concern local generation, but the risks (a mechanically rotating generator versus inverter-coupled PV) and therefore the normative points of attention differ.