ElectricityElectrical Installations IV (year 7) · 144 h

Installations in special buildings

A house is wired with general rules. A cinema, a hospital, a swimming pool or a paint plant is not: in those places the same electrical fault has different consequences —panic, a patient with an exposed heart, water, an atmosphere that explodes— and so the regulations add requirements of their own. This page goes through what changes in each case, how safety circuits are supplied, how a medical room is solved, where low-current systems and extra-low voltage fit, and how the switchboards and earthing of a large building are put together.

Special locations IP rating Medical IT system Safety circuits SELV Low-current systems

01What makes a place “special”

The general rules of electrical installations and of electrical protection assume an adult user with dry skin who knows the place and can walk out of it. A location is special when one of those assumptions stops holding: there are many people who do not know the building, the body is wet or connected to equipment, the environment attacks the material or a single spark is enough to start a fire. The Argentine regulation for installations in buildings (AEA 90364) devotes its Part 7 to the particular rules for those places: they do not replace the general ones, they tighten them.

Place	What makes it special	What changes in the installation
Places of public assembly (cinemas, theatres, shopping centres, stadiums, auditoriums)	Many people who do not know the building, panicked evacuation, smoke.	Emergency and escape lighting, safety circuits with their own supply, cables that do not spread flame and emit little smoke, switchboards with restricted access.
Medical rooms	The patient may have broken skin or a catheter: currents far below those felt on intact skin are already dangerous. And the power cannot be cut mid-operation.	IT system with isolating transformer and monitor, strict equipotential bonding, backup supply.
Damp and wet locations (bathrooms, industrial kitchens, laundries, cold rooms)	Humidity lowers body resistance and degrades insulation.	Higher IP rating, 30 mA RCD, zones with restrictions, corrosion-resistant conduit and fittings.
Swimming pools and fountains	Wet, barefoot body in contact with water.	Zones around the basin with increasingly restricted equipment, safety extra-low voltage for anything submerged, supplementary equipotential bonding.
Explosive atmospheres (petrol stations, silos, paints, solvents)	Gas, vapour or dust in explosive proportion: a spark or a hot surface is enough to start the explosion.	Area classification into zones, Ex-protected equipment, seals in the conduits, work under permit.
Outdoors and harsh environments	Rain, dust, sun, impacts, corrosion, storms.	Enclosures with suitable IP and IK ratings, UV-resistant materials, surge protection.
Construction sites	Temporary installation, damp, impacts, staff that changes every day.	Closed site switchboards, 30 mA RCD on every socket, cables fit for rough use, wiring protected from traffic.

The IP rating: how much an enclosure can take

The IP code (ingress protection) of IEC 60529 —in Argentina, IRAM 2444— says what an enclosure keeps out. It has two digits: the first against solids (a finger, a tool, dust); the second against water. An “X” means “not tested for that aspect”.

1st digit	Protects against
2	Solids larger than 12.5 mm: a finger.
3	Solids larger than 2.5 mm: tools and thick wires.
4	Solids larger than 1 mm: thin wires, screws.
5	Dust in an amount that does not impair operation.
6	Completely dust-tight.

2nd digit	Protects against
1 to 3	Drips and light rain (up to 60° from the vertical).
4	Splashing from any direction.
5	Water jets from an ordinary hose.
6	Powerful jets (pressure washing, heavy seas).
7	Temporary immersion (up to 1 m for 30 min).
8	Continuous immersion, under the conditions set by the manufacturer.

So an IP54 cabinet withstands a limited amount of dust and splashing: it will do in a workshop; an industrial kitchen washed with a hose calls for IP65 or more. The IP code says nothing about impacts: that is what the IK code is for, a separate rating.

The idea to keep

A special building does not have “another kind of electricity”: it has the same principles with less margin for error. Each requirement answers a concrete question: what if the voltage disappears? what if it shows up where it should not? what if the material gets wet, knocked or hot? The answer to the first calls for backup supplies; to the second, RCDs, equipotential bonding and an earthing system; to the third, an IP rating and suitable materials.

Part 7 section numbers are not quoted from memory

Each special location has its own section within Part 7 of AEA 90364. The exact values —zone distances, disconnection times, minimum IP ratings— are always taken from the current edition of the regulation and from the municipal building ordinances, not from a table in lecture notes. This page teaches the criterion; the standard fixes the number.

02Safety circuits and their own supply

A special building has two kinds of circuits. Normal circuits supply what makes the place run: general lighting, sockets, air conditioning. Safety circuits supply what protects people when something goes wrong: emergency and escape lighting, fire pumps, evacuation lifts, stairwell pressurization, alarms and detection. The latter must keep working when the mains fails, and for that they have an own supply.

Figure 1. Supply of a building with a safety source, animated. With the mains present (blue) everything works; when it is lost, the emergency lighting switches to its battery (violet) while the generating set starts; when the set takes the load (amber) the safety circuits return and the non-essential ones are shed.

No interruption

UPS or batteries with an inverter. For whatever cannot tolerate a single instant without power: alarm panels, intensive-care equipment, IT systems.

Short interruption

Central battery or self-contained luminaires. They cover emergency lighting during the evacuation, with an autonomy set by the applicable regulation.

Interruption of seconds

Generating set with automatic changeover (see Power generation and generating sets). It sustains the pumps, the lifts and the safety circuits for a long time.

Three rules repeat in every building with safety circuits:

Independence. The safety supply has its own line from the main switchboard, with its own protection, so that a fault in a normal circuit does not leave it without power.
Fire resistance. The cables of safety circuits are chosen of the type that keeps service during a fire (circuit integrity test, IEC 60331 series) and are run along protected routes. On escape routes, the other cables should preferably be low-smoke zero-halogen.
Supervision. A safety circuit that is not known to work is useless: self-contained luminaires and generating sets are tested periodically, with a record (see Maintenance of electrical components and Maintenance management).

Emergency and escape lighting

Two functions are distinguished. Escape-route lighting lets people find and follow the evacuation route to an exit; anti-panic lighting of large open areas (halls, lobbies) lets them recognize the place and reach the route without losing their bearings. Both are accompanied by exit signs, which also light up when the mains fails. The minimum level along the route axis and how long it must last —the autonomy— are set by the health-and-safety regulation and the local building code. Working out how many luminaires are needed is the subject of Lighting installations; the circuit that supplies them is solved in section 06.

Safety circuits are not “switched off to work”

Cutting the supply of emergency lighting or fire detection to work more comfortably leaves the building unprotected for as long as it lasts. If they must be worked on, the person in charge is told, it is recorded and it is restored before leaving, with the functional test done. And safety wiring routes are not shared with normal circuits.

03Medical rooms

Dry human skin has a resistance of thousands of ohms; skin pierced by a cannula, an electrode or a catheter no longer protects it. When current has a direct path to the heart, microamperes are enough —currents that on intact skin are not even felt— to disturb the heart rhythm. On top of that, in an operation or in intensive care the power cannot be cut at the first fault. Rooms where the patient is connected to equipment that acts on the heart or sustains vital functions (operating theatres, intensive care, haemodynamics) are solved with a system different from that of a house.

Figure 2. Medical IT system, animated. The isolating transformer leaves the secondary circuit floating with respect to earth. With sound insulation (1) no current flows through earth; the first fault (2) lets only milliamperes through and just makes the monitor sound; a second fault (3) closes a short circuit and only then does the protection act.

This is the IT system from the table in Electrical protection applied to a medical room:

A single-phase isolating transformer of limited power galvanically separates the room from the mains: no point of the secondary is connected to earth.
An insulation monitoring device (IMD) continuously measures the insulation resistance between each live conductor and earth. When it falls below a threshold —of the order of 50 kΩ in common devices— it raises a visual and audible alarm. The alarm is never silenced or disconnected: the faulty equipment is located and removed as soon as the procedure allows.
The first fault to an exposed conductive part does not cut the power: the current can only return through the insulation of the other conductor, which is very high, so it is small.
If a second fault appears on the other conductor before the first is repaired, the two join through the exposed conductive parts and a short circuit results, which the protections clear. That is why the monitor is not an ornament: it is what prevents getting there.

Example · How much current the first fault lets through

The transformer secondary is 220 V. The monitor has just alarmed because one conductor touched the casing of an item of equipment (a bolted fault) and the other conductor has an insulation of 50 kΩ to earth. The fault current returns through that insulation:

I_{f} = \frac{U}{R_{ins}} = \frac{220 V}{50 kΩ} = 4.4 mA

That is 4.4 mA: it cuts nothing and the equipment keeps working; and since all the exposed conductive parts of the room are bonded together, no dangerous potential difference appears either. By comparison, in a TT system of a house the same bolted fault is cleared only by the RCD. Check by another route: the conductance of the insulation is $\frac{1}{50 kΩ} = 20 µS$ and $220 V \cdot 20 µS = 4.4 mA$ .

Equipotential bonding and backup supply

Even with sound insulation, two metal objects in the room at different potentials can pass current through a patient who touches them. That is why in a medical room all exposed conductive parts (equipment casings, operating table, earth sockets) and all extraneous conductive parts (pipes, metal structure, frames) are connected to a room equipotential bar with short, low-resistance conductors, and that bar is connected to the building earthing. The aim is not to “bring to zero” the voltage with respect to earth but that between any two things a patient can touch there be no difference.

These rooms also have a backup supply: a generating set with automatic changeover and, for critical equipment, a UPS. And the room's circuits are not mixed with those of other areas: each room has its own switchboard, with its own transformer, monitor and protections.

A monitor alarm is a work order, not a nuisance

Silencing the alarm, bridging the monitor or removing its earth connection “so it stops bothering” eliminates the only protection of the system: the next fault is already the second. The equipment is identified (unplugging one item at a time), taken out of service and only then is the alarm reset.

04Low-current systems and extra-low voltage

Low-current systems (corrientes débiles) are the signal and communication installations that do not deliver power to a load: alarms, data, telephony, video, door entry. They work at extra-low voltage and with small currents. Being “weak” does not make them secondary: in a special building they are the ones that give warning, release doors and record what happens, and they must keep working like the power does in an emergency.

System	What it does	Installation particulars
Fire detection and alarm	Smoke and heat detectors, call points, sounders, controlled by a panel.	Supervised loops; fire-resistant and generally shielded cables; own battery supply. International references in common use: NFPA 72 and the EN 54 series.
Intruder alarm	Sensors on doors, windows and rooms; sirens; communicator.	Each zone is closed with an end-of-line resistor: that way the panel tells a cut or short circuit of the cable from a normal opening. Battery backup.
Data and telephony	The building's computer network and telephony (structured cabling).	Twisted-pair cable with a permanent link of up to 90 m plus 10 m of patch cords; optical fibre in backbones between floors and buildings; earthed racks.
CCTV	Cameras and recorders.	IP cameras powered over the same data cable (PoE, IEEE 802.3af/at/bt) or at extra-low voltage from a central supply; voltage-drop calculation matters.
Door entry and access control	Intercom, electric locks, readers.	An electromagnetic lock on an evacuation route must open if voltage is lost or on a fire alarm.

For the data side see Data communications; for the door phone and bells of a house, Residential electrical design.

Keeping signal apart from power

The rule is one: low-current systems do not share a route with power, unless the tray has a continuous divider (see figure 3). There are two reasons. The first is noise: a power cable running parallel to a data cable induces a voltage in it that degrades the signal, the more so the longer the parallel run and the more current it carries. The second is safety: if a power cable is damaged it must not be able to put voltage on a signal circuit that someone is touching. So they are routed separately, power is crossed at 90° and, where they run parallel, a distance is kept that depends on the length of the run and on what the structured-cabling standard establishes.

Figure 3. Power and low-current wiring routes. On the left, a tray with a continuous earthed metal divider; on the right, the 90° crossing between a power cable and a data cable, which minimizes coupling.

Extra-low voltage: SELV, PELV and FELV

Extra-low voltage does not exceed 50 V a.c. or 120 V ripple-free d.c. Below those values, contact with live parts under normal conditions is not dangerous; but a low voltage is not enough: how it is generated matters.

System	How it is obtained	Where it is used
SELV (MBTS)	Safety source (safety transformer or battery). The circuit is not earthed and is separated from the rest: no dangerous voltage can appear from a fault on the power side. Plugs must not fit sockets of another voltage.	Higher-risk places: submerged swimming-pool lighting (12 V a.c., 30 V ripple-free d.c.), bathrooms, toys, tools in conductive enclosures.
PELV (MBTP)	Same as SELV but the circuit, or its exposed conductive parts, may be earthed.	Control and signal systems where earthing is necessary for their function.
FELV (MBTF)	Extra-low voltage without the safety separation: obtained from an ordinary transformer or an autotransformer.	Control and signalling. It does not offer the protection of SELV: it is protected like a low-voltage circuit.

At 12 or 24 V every tenth of a volt counts: the current for the same power is very large and voltage drops weigh, in proportion, far more than at 220 V. A SELV circuit's cross-section is sized by voltage drop before current rating (section 06).

05Switchboards, discrimination, wiring routes and earthing

Switchboards of a large building

A large building does not have “a switchboard”: it has a main low-voltage switchboard (TGBT) that receives the service connection or the transformer, and from it sub-boards by floor, sector or service. Safety circuits leave from a board of their own, also supplied by the set. Each board is built to the standard for low-voltage assemblies (IEC 61439 series), with its rated short-circuit withstand stated, and is placed in an accessible location, with restricted access and ventilated. For large ones the form of internal separation (1 to 4) is specified: how far the busbars, functional units and terminals are separated from each other, so that one can be worked on without exposure to the others.

Switching is also designed for the emergency: an accessible main isolator, local and remote control of the safety circuits, emergency stop of machines and isolators that allow one part to be worked on without cutting the building. For protection and short-circuit calculations see Electrical protection and AC power.

Discrimination in a cascade of boards

In a building with many levels, a fault in a ground-floor socket must not leave the whole building without power. Discrimination (selectivity) is achieved with steps: protections are chosen so that, on a fault, the nearest one acts and those upstream do not. With circuit-breakers, rated current and curve are stepped; with RCDs, a selective (time-delayed, type S) device of higher sensitivity is placed at the head —for example 300 mA— and instantaneous 30 mA ones downstream. A common rule of thumb is that the upstream device should have at least three times the sensitivity of the next one. In safety circuits discrimination is indispensable: a fault in a luminaire must not cut the fire pump.

Wiring routes

In a large building cable trays or trunking are used for the backbones and conduit or wall trunking for the final runs. The criteria added in a special building are few but important: routes that pass through fire walls or slabs are sealed with a material that restores that fire resistance; safety routes follow protected paths; in damp or corrosive places materials and fittings that do not degrade are used; and a reserve of space for extensions is left, without filling the route to its limit. For cables and their selection see Electrical materials.

Earthing and equipotential bonding

All the exposed conductive parts of a building are connected, through protective conductors, to the main earth bar, and that to the electrode (rods, mesh, tape). The extraneous conductive parts that enter the building —metal water or gas pipes, structure, ducts— are joined to the same bar with the main equipotential bonding; in special locations supplementary equipotential bonding is added, local, joining together everything someone could touch with the hand at the same time. Shields of data cables and racks connect to that same network, not to a “separate earth” that would create potential differences.

In a TT system the earth resistance value is set by the RCD: the fault current that makes the device act, as it flows through the earth, must not raise the exposed conductive parts above the conventional touch-voltage limit $U_{L}$ :

R_{A} \leq \frac{U_{L}}{I_{Δ n}}

R_A: earth resistance of the exposed conductive parts · U_L: touch-voltage limit · I_Δn: RCD sensitivity

The table gives the maximum value for the usual touch-voltage limit of 50 V and for a stricter 24 V, adopted when the environment raises the risk (humidity, prolonged contact). It was calculated for two RCD sensitivities:

Sensitivity $I_{Δ n}$	$U_{L}$ = 50 V	$U_{L}$ = 24 V
30 mA	$\leq 1 667 Ω$	$\leq 800 Ω$
300 mA	$\leq 167 Ω$	$\leq 80 Ω$

That is why a 300 mA selective RCD at the head, useful for discrimination, demands an earth ten times better. Measuring earth resistance and insulation is explained in Earthing, insulation and generator testing.

06Worked example: emergency lighting of a corridor

Example · A 24 V SELV circuit for a 40 m corridor

An evacuation corridor in an office building has 8 emergency luminaires of 5 W at 24 V d.c., supplied from a 24 V central battery (two 12 V batteries in series) that must sustain them for 1 h. The lighting design has already fixed how many luminaires go and where. The cable cross-section and the battery capacity are required. Design data: at the end of discharge the battery delivers 21 V; the luminaires work down to 24 V − 20 % = 19.2 V; a voltage drop of 5 % is allowed; the farthest point is 40 m from the board (all the load is taken as concentrated there: a calculation on the safe side); copper at ambient temperature has $κ = 56 m/(Ω·mm²)$ .

1) Current in the worst case. The luminaires draw almost constant power: as the battery falls, the current rises. It is calculated with the final voltage:

I = \frac{P}{V_{\min}} = \frac{40 W}{21 V} = 1.905 A

2) Allowable drop and minimum cross-section. 5 % of 24 V is the maximum drop. The circuit is out and back, so the current path is $2 \cdot L$ :

{Δ V}_{\max} = 0.05 \cdot 24 V = 1.2 V

S \geq \frac{2 \cdot L \cdot I}{κ \cdot {Δ V}_{\max}} = \frac{2 \cdot 40 m \cdot 1.905 A}{56 m/(Ω·mm²) \cdot 1.2 V} = 2.27 mm²

The next commercial cross-section up is 2.5 mm².

3) Check. With 2.5 mm² the actual drop is:

Δ V = \frac{2 \cdot 40 m \cdot 1.905 A}{56 m/(Ω·mm²) \cdot 2.5 mm²} = 1.09 V = 4.5 % of 24 V

By another route: the resistance of one conductor is $\frac{40 m}{56 \cdot 2.5} = 0.286 Ω$ , the out-and-back loop is $0.571 Ω$ , and $0.571 Ω \cdot 1.905 A = 1.09 V$ . They agree. The voltage at the farthest luminaire, at the end of discharge, is $21 V - 1.09 V = 19.91 V$ , above the 19.2 V minimum: it complies. With 1.5 mm² the drop would be:

Δ V = \frac{2 \cdot 40 m \cdot 1.905 A}{56 m/(Ω·mm²) \cdot 1.5 mm²} = 1.81 V = 7.6 %

That is 7.6 %, above the 5 % design limit: it does not comply. So the 1.5 mm² cable, which would be plenty for current, is insufficient in this case.

4) Battery. With a factor of 1.25 for ageing and temperature:

C = I \cdot t \cdot k = 1.905 A \cdot 1 h \cdot 1.25 = 2.38 Ah

The next commercial battery up is adopted: 7 Ah (12 V each, two in series). The circuit is protected with a 4 A d.c. fuse placed next to the battery: the short-circuit current of a lead battery exceeds a hundred amperes and the 2.5 mm² cable would not withstand it. Wiring in a route of its own, separate from the normal circuits.

What the example teaches

In extra-low voltage the factor in command is the voltage drop, not the current rating: 1.9 A passes without trouble through 1.5 mm², but the drop over 40 m is already 7.6 %. And it is always calculated at the worst moment: discharged battery, farthest luminaire.

07In the workshop

Practice 1 · IT system mock-up

Materials: a 220/220 V isolating transformer of 100 to 300 VA, two 47 kΩ resistors (2 W or more, insulated for 500 V), an a.c. milliammeter, a multimeter, a 40 W lamp with its holder, a board with a circuit-breaker and a 30 mA RCD upstream, and cables with safety plugs.

Steps: (1) Feed the primary through the RCD. (2) With the multimeter, measure between each secondary terminal and earth: record it. (3) Connect the lamp between one secondary terminal and earth: it must not light, because there is no return path. (4) Connect a 47 kΩ resistor between each secondary terminal and earth, with the milliammeter in series in one of them, and measure the current.

Safety: the secondary is at 220 V even though it “floats”: the whole set-up is built with the transformer unplugged and measured with hands away from the terminals.

What is measured and acceptance criterion: the current must agree, within ±10 %, with $\frac{220 V}{94 kΩ} = 2.34 mA$ . Explain why the lamp does not light and why, with the simulated fault, the upstream RCD does not trip.

Practice 2 · Surveying the safety of a building

Materials: plans of the school or of the assigned building, torch, camera, survey form, stopwatch.

Steps: walk the evacuation routes and note: number and position of emergency luminaires and exit signs; IP rating marked on cabinets in damp or outdoor areas; low-current routes relative to power routes; state of the board (access, labelling, protections, free of obstructions). Test a self-contained luminaire: with the building in service, cut its supply and time how long it keeps the light.

Acceptance criterion: the form identifies each non-conformity with its location and a photo, and the tested luminaire holds the light for the whole autonomy declared by the manufacturer.

Practice 3 · Voltage drop in an extra-low-voltage circuit

Materials: a 24 V d.c. supply (bench or with a safety transformer and rectifier), two lengths of two-core cable of 1.5 mm² and 2.5 mm², 40 m each (or another known length), a resistive load or 24 V lamps drawing around 2 A, and two multimeters.

Steps: (1) Measure the supply voltage under load, at its terminals. (2) Connect the load at the end of the 1.5 mm² run and measure the voltage at the load end and the current. (3) Repeat with the 2.5 mm² one. (4) Calculate the drop with the formula of the example and compare it with the measured one.

Acceptance criterion: the measured and calculated drops differ by less than 0.15 V; the report states which of the two cables would meet 5 % for the measured current.

08Common mistakes

Mistake	Consequence
Wiring a special location with the criteria of a house	The requirement that justifies the location is left out. It is found at inspection or, worse, in the accident.
Silencing or disconnecting the insulation monitor of a medical room	The next fault is already the second: short circuit and loss of power mid-operation.
Taking an equipotential connection “from the pipe” and not checking continuity	A loose or painted joint leaves a potential difference exactly where there should be none.
Using emergency luminaires without testing autonomy	The battery ages and after two years lasts minutes. It is discovered when the power fails.
Sharing the safety wiring route with normal circuits	A fire or fault in the normal circuit drags down the safety one.
Running data cables tight against and parallel to power cables	Noise, network drop-outs, intermittent errors that are very hard to find.
Sizing a 24 V circuit by current rating only	Excessive voltage drop: luminaires that do not light or equipment that resets at the end of discharge.
Using a 300 mA RCD “because it works” without measuring the earth	The earthing does not meet the maximum value for that RCD: exposed conductive parts can sit at a dangerous voltage.
An electromagnetic lock that does not open on loss of power or on a fire alarm	People trapped during evacuation.
Opening or connecting live equipment in an area classified as explosive	A spark is enough to start the explosion.

09Self-assessment

What is the difference between a normal circuit and a safety circuit?

The normal one supplies the building's everyday operation; the safety one protects people when something fails (emergency lighting, fire pumps, evacuation lifts, detection). The second has its own supply (battery, UPS, set), an independent line, fire-resistant cables and is tested periodically.

What does it mean for a cabinet to be IP65?

The first digit, 6, indicates it is completely dust-tight; the second, 5, that it resists water jets from an ordinary hose. It suits a place washed with a hose, though it does not withstand immersion.

Why does an operating theatre use the IT system and what does the insulation monitor do? How much current does a first fault let through at 220 V and 50 kΩ?

The IT system leaves the circuit floating with respect to earth, so the first fault does not cut the power and no dangerous current flows: $220 V / 50 kΩ = 4.4 mA$ . The monitor measures insulation all the time and warns before there is a second fault, which would be a short circuit.

Why are power and low-current routes kept separate?

For noise (power induces voltage in signal cables and degrades the data) and for safety (a power fault must not reach a signal circuit). Separate routes or trays with a continuous divider are used, and crossings are made at 90°.

What is SELV and what are its limits?

It is safety extra-low voltage: up to 50 V a.c. or 120 V d.c. ripple-free, obtained from a safety source, with no earthing of the circuit and separated from the rest. PELV allows earthing; FELV has no separation and is protected like a low-voltage circuit.

With a 30 mA RCD and a touch-voltage limit of 24 V, what maximum earth resistance does a TT system allow?

$\frac{24 V}{0.03 A} = 800 Ω$ . With 300 mA the value drops to 80 Ω: hence a selective head-of-line RCD requires a better earth.

In the corridor example, what happens if 1.5 mm² cable is used?

The drop is $\frac{2 \cdot 40 \cdot 1.905}{56 \cdot 1.5} = 1.81 V$ , that is 7.6 % of 24 V: it exceeds the 5 % design limit. In extra-low voltage the drop commands, not the current rating.

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