Part 4 of the 17th Edition details the methods and applications of protection for safety, and consideration of these details must be made as part of the design procedure.
Areas that the designer needs to address are:
- Protection against electric shock.
- Protection against thermal effects.
- Protection against overcurrent.
- Protection against overload.
- Protection against fault current.
- Protection against undervoltage.
- Protection against overvoltage.
- Requirements for isolation and switching.
Protection against electric shock
There are two ways that persons or livestock may be exposed to the effects of electric shock, these are:
1. By touching live parts of electrical equipment.
2. By touching exposed-conductive parts of electrical equipment or systems, which have been made live by fault.
One method used to protect against contact with live parts is to insulate them in enclosures and/or place them behind barriers. In order to ensure that such protection will be satisfactory, the enclosures/barriers must conform to BS EN 60529, commonly referred to as the Index of Protection (IP) code. This details the amount of protection an enclosure can offer to the ingress of mechanical objects, foreign solid bodies and moisture. Also, protection for wiring systems against external mechanical impact needs to considered. Reference should be made to BS EN 62262, the IK code.
Protective earthing, protective equipotential bonding and automatic disconnection in case of a fault is the most common method of providing Fault protection.
The disconnection times for final circuits not exceeding 32A is 0.4 s and for distribution circuits and final circuits over 32A is 5 s. For TT systems these times are 0.2 s and 1 s.
The low impedance path for fault currents, the earth fault loop path, comprises that part of the system external to the installation, i.e. the impedance of the supply transformer, distributor and service cables Ze, and the resistance of the line conductor R1 and circuit protective conductor (cpc) R2, of the circuit concerned.
The total value of loop impedance Zs is therefore the sum of these values:
Zs = Ze + (R1+R2) Ω.
It must be noted that the actual value of (R1+R2) is determined from:
Tabulated value of (R1+R2) x Circuit length x Multiplier
1000
The multiplier corrects the resistance at 20°C to the value at conductor operating temperature.
The designer obviously has some measure of control over the values of R1 and R2 but the value of Ze can present a problem when the premises, and hence the installation within it, are at drawing-board stage. Clearly Ze cannot be measured, and although a test made in an adjacent installation would give some indication of a likely value, the only recourse would either be to request supply network details from the Distribution Network Operator (DNO) and calculate the value of Ze, or use the maximum likely values quoted by the DNOs, which are:
TT system: 21 Ω.
TN-S system: 0.8 Ω.
TN-C-S system: 0.35 Ω.
But these values are pessimistically high and may cause difficulty in even beginning a design calculation.
Supplementary bonding is used as Additional protection to Fault protection and required under the following conditions:
1. When the requirements for loop impedance and associated disconnection times cannot be met (RCDs may be installed as an alternative).
2. The location is an area of increased risk such as detailed in Part 7 of the Regulations, e.g. bathrooms, swimming pools..
Protection against thermal effects
The provision of such protection requires, in the main, a commonsense approach. Basically, ensure that electrical equipment that generates heat is so placed as to avoid harmful effects on surrounding combustible material. Terminate or join all alive conductors in approved enclosures, and where electrical equipment contains in excess of 25 litres of flammable liquid, make provision to prevent the spread of such liquid, for example a retaining wall round and oil-filled transformer.
Protection against overcurrent
The term overcurrent may be sub-divided into:
1. Overload current.
2. Fault current. This is further sub-divided into:
a) Short-circuit current (between live conductors)
b) Earth fault current (between line and earth)
Overloads are overcurrents occurring in healthy circuits and caused by for example, motor starting, inrush currents, motor stalling, connection of more loads to a circuit than it is designed for..
Fault currents, on the other hand, typically occur when there is mechanical damage to circuits and/or accessories causing insulation failure or breakdown.
Clearly, significant overcurrents should not be allowed to persist for any length of time, as damage will occur to conductors and insulation.
Protection against overload
Protection devices used for this purpose have to be selected to conform with the following requirements:
1. The nominal setting of the device In must be greater than or equal to the design current Ib: In ≥ Ib
2. The current-carrying capacity of the conductors Iz must be greater than or equal to the nominal setting of the device In: Iz ≥ In
3. The current causing operation of the operation of the device I2 must be less than or equal to 1.45 times the current-carrying capacity of the conductors Iz: I2 ≤ 1.45 x Iz
Overload devices should be located at points in a circuit where there is a reduction in conductor size.
Protection against fault current
Short-circuit current
When a "bridge" of negligible impedance occurs between live conductors the short-circuit current that could flow is known as the "prospective short-circuit current" (PSCC), and any device installed to protect against such a current must be able to break and in the case of a circuit breaker, make the PSCC at the point at which it is installed without the scattering of hot particles or damage to surrounding materials and equipment. It is clearly important therefore to select protective devices that can meet this requirement.
When an installation is being designed, the PSCC at each relevant point in the installation has to be determined, unless the breaking capacity of the lowest rated fuse in the system is greater than the PSCC at the intake position. For supplies up to 100A the supply authorities quote a value of PSCC, at the point at which the service cable is joined to the distributor cable, of 16kA. This value will decrease significantly over only a short length of service cable.
Earth fault current
We have already discussed this topic with regard to shock risk, and although the protective device may operate fast enough to prevent shock, it has to be ascertained that the duration of the fault, however small, is such that no damage to conductors or insulation will result. This may be verified in two ways:
1. If the protective conductor conforms to the requirements of Table 54.7 (IEE Regulations).
2. The c.s.a (conductor sectional area) of the protective conductor is not less than that calculated by using the formula:
S =
|
Ö(I²t)
| |
k
|
where
| S is the minimum protective conductor cross-sectional area (mm2) | |
| I is the fault current (A) | ||
| t is the opening time of the protective device (s) | ||
| k is a factor depending on the conductor material and insulation, and the initial and maximum insulation temperatures. | ||
Discrimination
It is clearly important that, in the event of an overcurrent, the protection associated with the circuit in question should operate, and not other devices upstream. It is enough to simply assume that a device one size lower will automatically discriminate with one a size higher. All depends on the "let-through" energy of the devices. If the total "let-through" energy of the lower rated device does not exceed the pre-arcing "let-through" energy of the higher rated device, then discrimination is achieved.
Protection against undervoltage
In the event of a loss of or significant drop in voltage, protection should be available to prevent either damage or danger when the supply is restored.
This situation is most commonly encountered in motor circuits, and in this case the protection is provided by the contactor coil via the control circuit.
If there is likely to be damage or danger due to undervoltage, standby supplies could be installed and, in the case of computer systems, uninterruptible power supplies (UPS). Switching on of very large loads can have the effect of causing such undervoltages.
Protection against overvoltage
Overvoltages can be caused by:
- Transient overvoltages of atmospheric origin.
- Switching surges within the installation.
Requirements for isolation and switching
An isolator is, by definition, a mechanical switching device which provides the function of cutting off, for reasons of safety, the supply to all or parts of an installation, from every source.
A switch is a mechanical switching device which is capable of making, carrying and breaking normal load current, and some overcurrents. It may not break short-circuit currents.
Isolators are off-load devices and switches are on-load devices.
Common devices are circuit breakers, RCDs, isolating switches, plugs and socket outlets, plug fuses..
No comments:
Post a Comment