Measures of protection against fire risk with RCDs
RCDs are very effective devices to provide protection against fire risk due to insulation fault. This type of fault current is actually too low to be detected by the other protection (overcurrent, reverse time). For TT, IT TN-S systems in which leakage current can appear, the use of 300mA sensitivity RCDs provides a good protection against fire risk due to this type of fault. An investigation has shown that the cost of the fires in industrial and tertiary buildings can be very great. The analysis of the phenomena shows that fire risk due to electricity is linked to overheating due to a bad coordination between the maximum rated current of the cable (or isolated conductor) and the overcurrent protection setting. Overheating can also be due to the modification of the initial method of installation (addition of cables on the same support). This overheating can be the origin of electrical arc in humid environment. These electrical arcs evolve when the fault current-loop impedance is greater than 0.6 Ωand exist only when an insulation fault occurs. Some tests have shown that a 300mA fault current can induce a real risk of fire
RCDs are very effective devices to provide protection against fire risk due to insulation fault because they can detect leakage current (ex : 300 mA) which are too low for the other protections, but sufficient to cause a fire
Protection when exposed conductive parts are not connected to earth
(In the case of an existing installation where the location is dry and provision of an earthing connection is not possible, or in the event that a protective earth wire becomes broken).
RCDs of high sensitivity (y 30 mA) will afford both protection against indirect-contact hazards, and the additional protection against the dangers of direct-contact.
Determination of voltage drop
The impedance of circuit conductors is low but not negligible: when carrying load current there is a voltage drop between the origin of the circuit and the load terminals. The correct operation of a load (a motor, lighting circuit, etc.) depends on the voltage at its terminals being maintained at a value close to its rated value. It is necessary therefore to determine the circuit conductors such that at full-load current, the load terminal voltage is maintained within the limits required for correct performance.
methods of determining voltage drops, in order to check that:
* They comply with the particular standards and regulations in force
* They can be tolerated by the load
* They satisfy the essential operational requirements
Maximum voltage drop
Maximum allowable voltage-drop vary from one country to another. Typical values for LV installations are given below
A low-voltage service connection from a LV pubic power distribution network , lighting 3% - Other uses ( Heating & Power 5% )
Consumers MV/LV substation supplied from a public distribution MV systems , 6% / 8%
Maximum voltage-drop between the service-connection point and the point of Utilization ,
These voltage-drop limits refer to normal steady-state operating conditions and do not apply at times of motor starting, simultaneous switching (by chance) of several loads, etc. as mentioned in Chapter A Sub-clause 4.3 (factor of simultaneity, etc.). When voltage drops exceed the values , larger cables (wires) must be used to correct the condition.
The value of 8%, while permitted, can lead to problems for motor loads; for example:
* In general, satisfactory motor performance requires a voltage within ± 5% of its rated nominal value in steady-state operation,
* Starting current of a motor can be 5 to 7 times its full-load value (or even higher). If an 8% voltage drop occurs at full-load current, then a drop of 40% or more will occur during start-up. In such conditions the motor will either:
* Stall (i.e. remain stationary due to insufficient torque to overcome the load torque) with consequent over-heating and eventual trip-out
* Or accelerate very slowly, so that the heavy current loading (with possibly undesirable low-voltage effects on other equipment) will continue beyond the normal start-up period
* Finally an 8% voltage drop represents a continuous power loss, which, for continuous loads will be a significant waste of (metered) energy. For these reasons it is recommended that the maximum value of 8% in steady operating conditions should not be reached on circuits which are sensitive to under-voltage problems ,
Legal basis
In England and Wales, the Building Regulations (Approved Document: Part P) require that domestic electrical installations are designed and installed safely according to the "fundamental principles" given in British Standard BS-7671 Chapter 13. These are very similar to the fundamental principles defined in International Standard IEC – 60364-1 and equivalent national standards in other countries. Accepted ways for fulfilling this legal requirement include
* the rules of the IEE wiring regulations ( BS–7671 ), colloquially referred to as "the regs" (BS 7671: 2008, 17th Edition).;
* the rules of an equivalent standard approved by a member of the EEA (e.g., DIN/VDE 0100);
* guidance given in installation manuals that are consistent with BS 7671, such as the IEE On-Site Guide and IEE Guidance Notes Nos 1 to 7.
Installations in commercial and industrial premises must satisfy various safety legislation, such as the Electricity at Work Regulations 1989. Again, recognized standards and practices, such as BS 7671 "Wiring Regulations", are used to help meet the legislative requirements.
Commissioning Certificate
BS-5266 and the European Standard both require written declarations of compliance to be available on site for inspection. These consist of
1. Installation quality.
IEE regulations must have been conformed with and non-maintained fittings fed from the main circuit of the normal lighting system, as required in BS 5266
2. Photometric performance.
Evidence of compliance with light levels has to be supplied by the system designer.
3. Declaration of a satisfactory test of operation.
A log of all system tests and results must be maintained. System log books, with commissioning forms, testing forms and instructions should be provided by the installer.
On completion of the installation of the emergency lighting system, or part thereof, a completion
certificate should be supplied by the installer to the occupier/owner of the premises. The Building Control Department should insist upon a copy of this certificate which will be retained with the Building Regulations Authority.
Maintenance
Finally, to ensure that the system remains at full operational status, essential servicing should be defined. This normally would be performed as part of the testing routine, but in the case of consumable items such as replacement lamps, spares should be provided for immediate use.
Routine inspections and tests
Where national regulations do not apply, the following shall be met.
Because of the possibility of a failure of the normal lighting supply occurring shortly after a period of testing of the emergency lighting system or during the subsequent recharge period, all full duration tests shall wherever possible be undertaken just before a time of low risk to allow for battery recharge. Alternatively, suitable temporary arrangements shall be made until the batteries have been recharged.
The following minimum inspections and tests shall be carried out at the intervals recommended below. The regulating authority may require specific tests.
Daily
Indicators of central power supply shall be visually inspected for correct operation.
NOTE. This is a visual inspection of indicators to identify that the system is in a ready condition and does not require a test of operation.
Monthly
If automatic testing devices are used, the results of the short duration tests shall be recorded.
For all other systems the tests shall be carried out as follows:
a) Switch each luminaire and each internally illuminated exit sign to emergency mode so it uses the battery. This simulates a failure of the supply of the normal lighting and continue for a period sufficient to ensure that each lamp is illuminated.
At the end of this test period, the supply to the normal lighting should be restored and any indicator lamp or device checked to ensure that it is showing that the normal supply has been restored.
NOTE. The period of simulated failure should be sufficient for the purpose of this clause whilst minimising damage to the system components e.g. lamps. During this period, all luminaire's and signs shall be checked to ensure that they are present, clean and functioning correctly.
b) For central battery systems, the correct operation of system monitors shall be checked.
c) For generating sets, refer to the requirement of ISO 8528-12.
Annually
If automatic testing devices are used, the results of the full rated duration test shall be recorded.
For all other systems the following tests made:
a) each luminaire and internally illuminated sign shall be tested as per monthly test but for its full rated duration in accordance with the manufacturer's information;
b) the supply of the normal lighting shall be restored and any indicator lamp or device checked to
ensure that it is showing that normal supply has been restored. The charging arrangements should
be checked for proper functioning;
c) the date of the test and its results shall be recorded in the system logbook;
d) For generating sets, refer to the requirements of ISO 8528-12.