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1/4Schneider Electric - Electrical installation guide 2005

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E - Distribution in low-voltage installations 1 LV distribution schemes

1.4 Safety or emergency-services installations andstandby power supplies

Safety installations

Safety and emergency-services installations are governed by statutory regulations,for example concerning:

c Public buildings

c High-rise apartment blocks

c Premises in which people are employed (offices, shops, factories, etc.)

Such premises must be provided with a means of ensuring the safe evacuation ofpersonnel, including:

c Security and safety lighting

c Alarms and warning systems

c Automatic fire detection systems

c Fire-extinguishing systems

c Smoke evacuation systems

c

Air compressors for the pressure-operated fire-extinguishing systemc Water pumps to refill the fire extinguishing system

Apart from the general rules noted above, there are certain projects for which safetyregulations are related to a particular process (e.g. petro-chemical plants, cementworks, etc.) or services (e.g. tunnel lighting, airport runway lighting, etc.).

Standby reserve-power supplies

Among the many applications in which an interruption of power supply cannot betolerated, the following may be cited (see Fig. E18 ):

c Information technology installations (protection of data concerning insurance,banking, professional practices, governments)

c Industrial processes (continuity of feed material for continuous processing, boilerfeed-water pumps in power stations, paper

production, desalination plants)

c Food-processing industry (refrigeration plants, hatcheries)

c Telecommunications

c Scientific research

c Surgical operating theatres

c Ticketing, plane reservations, cash registers

c Military

It may be noted that where several emergency-services standby sources exist, theycan also be used as reserve-power sources, on condition that any one of them isavailable and capable of starting and supplying all safety and emergency circuits, andthat the failure of one of them does not affect the normal functioning of the others.

Choice and characteristics of reserve-power supplies

Apart from perceptible (albeit very brief) cuts in power supply, imperceptibleinterruptions of several milli-seconds are sufficient to interfere with the operation ofcertain equipment. As previously noted, UPS systems are essential in these cases,

and are used together with the reserve-power source to ensure the utmost security.

Fig. E17: Example of a High Quality Power installation

HV

LV

Diesel

Automatic

bypass

generator

UPS

The provision of safety and emergencyinstallations is a legal obligation

Standby reserve-power supplies are aneconomic necessity in numerous circumstances

where loss of supply would have far-reachingconsequences

Fig. E18: Examples of reserve power supplies: central storage battery (left) and diesel-generator sets (right)

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E - Distribution in low-voltage installations

Principal specifications

For economic reasons, the following requirements are often mandatory:

c Supply interruption is not tolerated:

v In information technology (IT) systemsv In continuous-process operations, except for loads of high inertia that can toleratean interruption in the order of 1 second

c Period for conserving data in information technology (IT) systems: 10 minutes

c Autonomy, often expressed as a backup time, is desirable for reserve-powersupplies installations; it is a function of the economics related to operation beyondthe minimum requirement for the safety of personnel

Specifications related to safety installations (see Fig. E19 )

National regulations covering safety installations contain a number of conditions tobe respected concerning their electric-power sources:

c Duration of an interruption: According to the case and the country, the followingchoices are generally imposed:

v No break

v A break of less than 1 second

v A break of less than 15 secondsc Autonomy demanded for the reserve-power source: In general it corresponds tothe time necessary to complete all operations governing the safety of human lives,for example, the time to evacuate a public building (1 hour minimum). In largeapartment blocks, the autonomy of the source must be 36 hours, or more.

Fig. E19: Table showing the choice of reserve-power supply types according to application requirements and acceptable supply-interruption times

Fields of application Programmable controllers Interruptible sequential Continuous

IT equipment process process

Telecommunications

Applications - Data bank - Cold-working sequence - Control and monitoring

- Process control and of process parameters

monitoring

Installation examples - IT services - Light machining - Nuclear

- Banking, insurance - Packaging assembly chain - Chemical

- Administration - Biological- Management system of - Thermal

production process - Heavy mechanical (high inertia)

Conditions

Allowable duration of break Zero c c

i 1 second c

i 15 seconds c(1)

i 15 minutes c(1)

Autonomy of source minimum 1 minute c(2)

and preferred 20 minutes c

1 hour c c

Permanent if economical

Solutions

Technique employed UPS with or without No-break generator or Permanent generator set

a generator set to take over start-up and take-overbefore the end of battery of load from the UPS

backup power

(1) Depending on economic circumstances

(2) Data-storage time limit

Choice and characteristics of different sources

The several possible solutions are characterised by their availability, i.e. immediateor delayed load pick-up time, and their autonomy, i.e. ability to supply the load for agiven period without human intervention (refilling fuel tanks for example). It is alsonecessary to take into account:

c Constraints imposed by the installation: in particular for specialised locations, and

according to the source(s) used

c Complementary equipment

c Operational constraints, e.g. according to manufacturer operating instructions orlocal statutory regulations, etc.

c Routine maintenance requirements, which could impose less than ideal restrictionsduring periods allotted to such work

1 LV distribution schemes

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E - Distribution in low-voltage installations 1 LV distribution schemes

An overall review of the many possibilities and associated constraints often leads toan optimum solution based on a UPS scheme associated with a standby dieselgenerator set. Batteries provide an uninterrupted supply of power during the start-up

and load pick-up time of the standby generator set (see Fig. E20 ).

Fig. E20: Characteristics of different sources

Emergency and/or

reserve power supply

Battery UPS Cold-start diesel Load pick-up (1) Generators in

continuous operation

Time required to supply load

Zero time (no break) c c c

1 second c

1 to 10 minutes (5) c

Total time for a changeover operation

Zero c c cRelated to the automaticchangeover scheme

adopted for each source

Installation constraints

Special location None. Unless batteries Special location

(type of battery) are vented type (vibrations, noise, access required for maintenance, fire protection)Special DC network Fuel tanks

Additional equipment (apart from protection and changeover devices)

Charger None. Unless additional Starter, by batteries or Inertia fly wheel Automatic synchronising

Regulator, indications batteries are required compressed air and clutch equipment

Operational mode and constraints

Special network Automatic Manual or automatic Automatic Permanent operating staffSystem losses Periodic startups Fixed maximum load

Frequent checking

Other parameters

Maintenance Periodic shut-downs None Periodic checks, but Minor mechanical Periodic checks, but

for checking and Unless batteries are minimal wear and very constraints only, except minimal wear and verymaintenance work vented type little upkeep required for clutch and coupling little upkeep required

Little upkeep required shaft

Life expectancy (3) 4 to 5 years (2) 4 to 5 years 1,000 to 10,000 hrs 5 to 10 years 10,000 hrs (or 1 year)( for sealed batter ies) or 5 to 10 years

Necessary x 2 if installation Typically 2 for 1 Batteries x 2 x 2 where security x 2 if the installation

redundancy (4) is permanent and 3 for 2 is important is permanent

Reliability factors (4) Constant checking is Integrated checks Mechanical and starter Mechanical, particularly Mechanical and

important (numerous batteries clutch assembly and synchronisation system

human errors) coupling shaft

(1) A motor-generator set running continuously and equipped with a heavy flywheelOn the loss of normal supply, the pick-up of load generally requires less than 1 second

(2) Longer if the battery is of the vented type

(3) Before requiring a major overhaul

(4) A study of safety requirements allows the definition of an optimal scheme

(5) According to whether the set is pre-heated or not

GM

Local generating sets

In certain installations, a power supply independent of the normal public service isneeded. In such cases, a local generator (usually driven by a diesel engine) isprovided and associated with a UPS. In this case the battery backup time of the UPSmust be sufficient to cover the time required to start the diesel engine and connectthe generator to the load.

The time required to effect a changeover from one source to the other depends onthe characteristics of the particular installation, such as start-up sequence of theengine, possible shedding of inessential loads, etc.

Connection is generally carried out at the main LV switchboard by means of anautomatic source changeover system such as the one illustrated in FigureE21next page.

The combination of a UPS and a localgenerating set is the optimum solution toensure long autonomy

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E - Distribution in low-voltage installations

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1 LV distribution schemes

During UPS operation under normal conditions, AC power flows into the rectifier

section and a very small portion of the DC power at the output of the rectifier is usedto keep the battery fully charged. The remainder of the DC power is converted intoclean AC power for the load.

In the event of a changeover from normal supply to reserve-power generator supply,it is important (particularly if the load to be supplied from the generator is largecompared to its rating) that damaging transient torques on the generator shaft andcouplings be avoided. Such torques occur for suddenly applied loads and are due tothe oscillating transient torque of the shaft and the steady load torque adding andsubtracting at the natural frequency of the shaft oscillations. To avoid thisphenomenon, the rectifier is controlled electronically to pass a low current initiallythat is then gradually increased until the load is supplied entirely by the generator.This operation lasts for 10-15 seconds.

UPS shutdown is also carried out gradually by similar controls on the rectifiercircuits.

A gradual application of load also avoids the possibility of large transient currents

and fluctuations in frequency, the latter being due to the inertia in the speed-regulation governor system of the prime mover.

The rectifier in the conversion system creates harmonic currents which generallymeans that the reserve-power generator has to be derated (i.e. an oversizedgenerator may have to be installed). This question should be discussed with theUPS manufacturer.

In the example shown in Figure E21, the output from the UPS is synchronised withthe input supply to the rectifier so that, in the event of overloading or failure of theUPS inverter, instantaneous closing of the static changeover switch will maintainsupply.

Fig. E21 : Example of a UPS / generating-set combination with an automatic changeover system

Protection and distributionequipment (complementary)

Possibletransformer (1)

Network 2

Static

changeover

switch

Battery

charger

InverterBatteryprotection

box

Network 1

Diesel

generator

(1) Necessary in some cases, e.g. for voltage matching

Normal power

source

Protection and

distribution equipment

(complementary)

Manual maintenance

bypassswitch

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