Three decisions settle a central safety power supply: the standard's own terms, a battery sized for its last day at its real discharge rate, and a room temperature written into the tender. Each one is a clause you can check.
Four decisions to carry into the next central battery specification you write.
EN 50171:2021 defines no "central battery system". Clause 3.19 bounds an LPS at 500 W for 3 h or 1500 W for 1 h, and everything above it is a CPS "with any rated power output". Clause 4 defines six operating modes, and a changeover mode must transfer within 0.5 s.
For a lead-acid bank, IEEE 485 converts the load to current at the end-of-discharge voltage, then applies ageing (1.25), a design margin and a temperature factor, and compares the result with the selected battery's own discharge table. In our worked example, 5 kW for 3 h needs 125 Ah at the 3 h rate against a naive 77 Ah, which selects 200 Ah blocs. That is an illustration under stated assumptions, not a universal result.
Heat can hide the loss: a warm VRLA cell reads slightly more capacity while it ages faster, so a hot or aged battery can still fail its duration test. EUROBAT halves life for every 10 K above 20 °C, so a 10-year design life is about 2.5 years in a 40 °C room: an estimate, not a guarantee. EN 50171 assumes a moderate climate unless the enquiry says otherwise, and in a Saudi plant room it has to.
EN 50172:2024 sets the periodic checks in 7.4.2 to 7.4.5, from a daily look at the CPS status indicators to a five-yearly illuminance check, and the logbook. The annual verification (7.4.4) repeats the 7.3 b) to f) inspections and functional checks: a full-duration test with every luminaire and sign checked, the CPS monitors, damage and dirt, rest and inhibit modes, and sign visibility. 7.4.1 also cites a 7.4.6 that the published text does not contain, an apparent cross-reference error with no corrigendum listed. EN 1838:2024 sets the light; IEC 60598-2-22 defines the luminaire. In KSA the SBC provisions cited in Episode 31 set at least 90 minutes for new buildings, not Europe's 1 h floor. A submittal that cites EN 50171 for monthly testing has cited the wrong document.
Every row names its clause and edition. EN 50171:2021 is an equipment standard: it sets how the central unit behaves, not how long it runs or how often it is tested.
| Requirement | Value | Source |
|---|---|---|
| The standard's terms | CPS; LPS is 500 W for 3 h or 1500 W for 1 h | EN 50171:2021, 3.18 and 3.19 |
| Operating modes | 6, in clauses 4.2 to 4.7 | EN 50171:2021, clause 4 |
| Changeover time | ≤ 0.5 s, changeover modes only | EN 50171:2021, 4.1 |
| Transfer trigger | mains below 0.85 of rated for over 0.5 s | EN 50171:2021, 6.5.1 |
| Output voltage | ±6% from 20 to 100% load; ±10% for 5 s on a step | EN 50171:2021, 6.6.2 |
| Recharge | 80% of operating time after 12 h | EN 50171:2021, 6.3.3 |
| Inverter overload | 120%, continuously | EN 50171:2021, 6.6.3 |
| Deep discharge | ≤ 0.2 A per 100 Ah; manual reset only | EN 50171:2021, 6.7 |
| Rated operating time | set by the user; at least 90 min in the SBC provisions cited for new buildings | EN 50171 5.2 e); SBC §1008.3.4 |
| Ambient | declare it if not Class 3K3 | EN 50171:2021, 5.2 h) |
| Ageing factor | 1.25, which is 1 / 0.8 | IEEE 485-2020, 6.3.4 |
| Design margin and temperature | 1.10 to 1.15; 1.03 for a 20 °C minimum (worked example) | TSEWG TP-4; Panasonic curve |
| VRLA life and heat | halves per 10 K above 20 °C (an estimate) | EUROBAT guide, 2022 |
| Battery room air | Q = 0.05 × n × Igas × C × 10-3 m³/h | EN IEC 62485-2:2018 |
| Periodic tests | daily, monthly, annual (7.3 b to f, incl. full duration), five-yearly | EN 50172:2024, 7.4.2 to 7.4.5 |
| End point and cut-off | sized to 189 V (1.75 V/cell); unit cuts off at 173 V | IEEE 485 example; Teknoware VOT115E |
| Circuit integrity | IEC 60331, at 830 °C; not IEC 60332 | IEC 60331-2:2018 |
From the battery's discharge table to the cable tray, in the order a design review meets them.
Panasonic publishes 200, 187, 169 and 118 Ah for one stationary VRLA bloc at the 20, 10, 3 and 1 hour rates. Across ten blocs in the family the 3 hour capacity averages 77% of nameplate and the 1 hour capacity 64%. Every emergency battery is a short-duration battery, so every one is bought at its worst discount. IEEE 485 handles it with the maker's constant-current table at a stated final voltage, not with Peukert's equation.
A central inverter or a DC luminaire driver draws constant power, so its current rises as the string sags. Our worked lead-acid example assumes 5 kW AC for 3 h through a 90% inverter, 108 cells and an end point of 1.75 V per cell, 189 V: 29.4 A, so 88 Ah at the 3 h rate. Ageing 1.25 (IEEE 485 clause 6.3.4) gives 110, a 1.10 margin 121, and a 1.03 factor for a 20 °C minimum room (interpolated from the bloc's own curve) 125 Ah. Panasonic's LC-P12200BP table at 25 °C to 10.5 V gives 169 Ah at 3 h, the LC-P12150BP only 119, so the 200 Ah bloc is selected; its watt table agrees (438 W per bloc needed, 608 W published). The handbook prints these as average values, so a procurement calculation runs on the offered battery's guaranteed data. Check the duty section by section: when non-maintained luminaires all start at once, the first minute can set the size.
The same bloc reads 102% of its capacity at 40 °C, so a warm battery can pass a test it would fail cool, while it ages faster. A hot or aged battery can still fail its duration test. EUROBAT halves VRLA life for every 10 K above 20 °C, a quarter at 40 °C, so the 10-year design life this bloc carries at 20 °C becomes about 2.5 years: an estimate on a rule, not a guarantee. Design life is a classification under reference conditions; service life is what the site gets. Quoting the rule from 25 °C, a warranty term, flatters every figure by 1.41. Oversizing buys capacity, never life. Teknoware's TKT75-78 CPS family is itself rated for 10 to 30 °C ambient, so the unit needs the cool room as much as the battery does.
EN IEC 62485-2 sizes hydrogen ventilation for lead-acid and NiCd as Q = 0.05 × n × Igas × C × 10-3 m³/h. For the selected 108 VRLA cells of 200 Ah that is 1.08 m³/h on float and 8.64 m³/h if the charger boosts at 2.40 V per cell, a free area of 30 or 242 cm² for EACH of the inlet and the outlet, assuming natural flow at 0.1 m/s with the openings on opposite walls or at least 2 m apart (HOPPECKE's installation manual). Interlock the charger with any fan. The same bank's nameplate energy is 43.2 kWh (216 V x 200 Ah), while the 3 h duty draws about 16.7 kWh: the difference is rate discount, end voltage, ageing and margin. Lithium is a different file: IEEE 485 sizes lead-acid only, IEEE 1679.1 evaluates lithium but excludes sizing, so size from the maker's data and BMS limits; IEC 62619 covers the battery and EN IEC 62485-5 the installation. In a lab test a 50 Ah LFP cell at 100% charge released 0.93 mol of hydrogen in thermal runaway. US model code context, not a Saudi requirement we have verified: IFC 2021 Table 1207.1.1 sets 20 kWh for lithium-ion and 70 kWh for lead-acid on nameplate energy, and 1207.3.1 asks for UL 9540 listing.
Two families. A direct-DC CPS feeds the circuits at 216 V DC in emergency, so its luminaires must accept both the AC normal supply and that DC (Teknoware states it for its units); a central inverter system gives AC in both states. Rate the luminaire for the outputs it will actually see. 189 V is the sizing end point of the worked example, the voltage the bank stays above through the rated 3 h at end of life. 173 V is the default deep-discharge cut-off of Teknoware's TKT75-78 (warning at 227 V, overvoltage at 255 V; user guide VOT115E): no usable duration lies below 189 V. So the last luminaire's minimum DC input must sit below 189 V less the cable drop, and its maximum above the float voltage, about 245 V, wherever the operating mode connects it to a charging string. For the cable, circuit integrity is IEC 60331, which keeps a circuit working at 830 °C. IEC 60332-1-2 only proves the cable does not spread flame, and EN 50577 says itself that it does not assess the supports.
The one head-to-head life-cycle study we located, BorWin5, 1800 luminaires over 30 years on an offshore platform, publishes euro totals; recomputed from its Table 6, central with manual testing comes out 13% cheaper than self-contained, but self-contained with automatic testing 22% cheaper, and central 3.5% dearer to install. Manual test labour, at EUR 86,400 a year, outweighed battery replacement at EUR 20,250. One site with offshore labour rates, not a rule. The paper also counts about 10,800 luminaire batteries replaced over 30 years against 20 central batteries; those are not like-for-like units, and its own maintenance section models a quarter of the packs a year, which would be more. Compare its end-of-life costs instead, EUR 62,359 against EUR 12,948, and the luminaires too small to hold a battery.
Five lines, each naming a document, a quantity and a unit. None of them can be answered with an adjective, which is the only test a tender clause has to pass.
a. A CPS to EN 50171:2021, mode named by clause, 230 V, 60 Hz, rated operating time not less than 90 min (SBC Section 1008).
b. Lead-acid sized to IEEE 485-2020 on the offered battery's guaranteed data, stated V/cell and minimum temperature, ageing 1.25, margin ≥ 1.10. Lithium: maker's data, IEC 62619.
c. Design life declared at 20 °C, battery room held at 20 to 25 °C, CPS unit rated for the room's maximum ambient.
d. Luminaires rated for the actual normal and emergency outputs, from end-of-discharge voltage less drop up to float, on IEC 60331 circuits.
e. Ventilation for the highest charging mode, EN IEC 62485-2 or EN IEC 62485-5; tests to EN 50172:2024 7.4.
Line c is the one most often left out. It decides how close a ten-year design life gets to ten years in service.
Clause 5.2 lists them: the load profile over the rated operating time, the maintained and non-maintained split, the permissible changeover time, the rated operating time, the battery type and position, and the ambient if it is not a moderate climate. NLC's central battery systems come in AC/DC and DC/AC configurations with IEC 62034 monitoring.
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The clause, the edition, and which document owns it.
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