Electrical Protection for Safety

ELECTRICAL PROTECTION FOR SAFETY

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..

CHP Technologies

CHP Technologies

CHP (Combined heat and power) is a specific form of distributed generation (DG), which refers to the strategic placement of electric power generating units. CHP enhances the advantages of DG by the simultaneous production of useful thermal and power output, thereby increasing the overall efficiency.

Summary of CHP Technologies:

- Gas turbines: gas turbines are typically available in sizes ranging from 500kW to 250MW and can operate on a variety of fuels such as natural gas, synthetic gas, landfill gas and fuel oils.

- Microturbines: which are small electricity generators. Microturbines use the fuel to create high-speed rotation that turns an electrical generator to produce electricity. In CHP operation, a heat exchanger referred to as the exhaust gas heat exchanger, transfers thermal energy from the microturbine exhaust to a hot water system HWS. Available models range in sizes from 30kW to 250kW.

- Reciprocating engines: spark ignition (SI) and compression ignition (CI) are the most common. SI engines use spark plugs with a high-intensity spark of timed duration to ignite a compressed fuel-air mixture within the cylinder. SI engines are available in sizes up to 5MW. Diesel engines, also called CI engines, are among the most efficient simple-cycle power generation options in the market. Reciprocating engines start quickly, follow load well, have good part-load efficiencies, and generally have high reliabilities.

- Steam turbines: that generate electricity from the heat produced in a boiler. The energy produced in the boiler is transferred to the turbine through high-pressure steam that in turn powers the turbine and generator. The capacity of commercially available steam turbine typically ranges between 50kW to over 250MW.

- Fuel cells: use an electrochemical or battery-like process to convert the chemical energy of hydrogen into hot water and electricity. There are currently five types of fuel cells under development. These include phosphoric acid (PAFC), proton exchange membrane (PEMFC), molten carbonate (MCFC), solid oxide (SOFC), and alkaline (AFC).

DHC District Heating and Cooling

DISTRICT AND COMMUNITY HEATING

DHC is an integrative technology that can make significant contributions to reducing emissions of carbon dioxide and air pollution and to increasing energy security.

The fundamental idea of DHC is simple but powerful: connect multiple thermal energy users through a piping network to environmentally optimum energy sources, such as CHP, industrial waste heat and renewable energy sources such as biomass, geothermal and natural sources of heating and cooling.

District heating is traditionally divided into two types namely that serving flats, apartments and housing, and that serving a variety of building owners that can include commercial, retail, industrial, residential and local authority. The former is normally called community heating and the latter district heating.

Both district and community heating in recent years have been associated with power generation in the form of combined heat and power plant (CHP) where the electrical power generated is used locally and any excess power supplied to the national grid.

In one community heating system using CHP, blocks of flats are heated from a central plant and the lifts, entrances, corridors and landings are powered and lit from the electricity generated. Waste incineration is used to generate heat and power in district heating as also is biomass.

The focus for district and community heating schemes is clearly heating, so, the heat load for the scheme will be the major component.

The effect on overall efficiency, when the CHP option is used, is to raise it from 30% to as high as 75%. It is necessary however for the generator to find a customer for the heat generated from the CHP plant. Community and district heating can also benefit from the application of CHP technology as electricity can be used to power the plant with the surplus sold to the national grid.

The two main investments in a district heating system are the heat production plant and the network. The heat production plant is a single investment; the cost of it depends on the total annual amount of heat load of the area. The investment in the pipe system, otherwise, is a question of the length of the pipe network within the area of the heat supply and therefore is dependent on two dimensions: thermal length and thermal width. Therefore it follows that the costs of the distribution network can vary appreciably for different network geometries and type of systems.

Factors for consideration:

The feasibility study for a new scheme will need to investigate and research a number of areas before and initial decision can be reached.

- Type and concentration of consumers: the greater the variety the better, and could include commercial, industrial, retail, residential, schools, libraries, museums, leisure centres...

- Base load consumer: this would provide a fairly constant and substantial requirement for heating throughout the year ensuring continuous use of plant. Another base load usually available is the need for hot water supply particularly in residential sector, hotels, hospitals, campings and sport centres.

- Consumer acceptance: a major factor is the charge for the service which needs to be at least 10% below the cost for running and maintaining individual boiler plant. Another factor would be the security of heat supply.

- Environmental factors: ideally the heat and power generating plant needs to be located near. However, this mitigates against the noise disturbance and pollution which may result.

- Energy utilization: a district heating proposal shows a clear advantage in its use of energy from fossil fuel, waste and renewable sources over local heat generation.

- Maintenance philosophy: one of the lessons learnt in early district heating schemes is the importance of getting the maintenance of plant and external pipework right. A sure way to achieve this is to engage the contractors responsible for the installation of the distribution mains, for example, in a maintenance contract, following completion.

CHOICE OF SYSTEM AND OPERATING PARAMETERS

Energy sources available: These would include the most likely energy sources for the scheme - fossil fuel available, type of waste and type of renewable energy.

Heat supply ratios: Assessment of approximate ratios of process, heating, hot water supply loads and power loads are needed for the feasibility study. This will have a direct bearing on the choice of plant.

Heating medium: The heating medium needs consideration. It is invariably water because of its high specific heat capacity and may be LTHW, MTHW or HTHW. Steam might be considered at the central plant because of its high latent heat.

Heat exchange: If high temperatures are employed from the central plant it will be necessary to reduce the temperature using heat exchangers at substations. Industrial consumers may want high temperature distribution for their process and space heating.

Consumer supply: The distribution pipework layout should seek to ensure a ring main arrangement, so that, if a consumer's supply fails, a temporary reconnection can be made quickly.

Basis for charging: There are three ways in which the consumer can be charged for energy use:
    - Flat rate, which avoids the use of metering equipment and hence its initial cost and maintenance (not recommended).
    - Service charge plus charge for energy actually used.
    - Charge for energy use that includes the service charge.
It is important for avoiding consumer concerns and providing a good and reliable service that the instrumentation installed must be carefully selected both in the accuracy of the metering equipment as in the recording process.

Future requirements: It needs to take in consideration future extensions to the scheme and make a strategic plan.

Diversity: The feasibility study will address the diversity factor that will be applied to the plant for sizing purposes. Clearly the central plant is not sized on the total net load when all consumers are on line. As a general rule, the lower the number and diversity of consumers connected to the scheme the higher will be the diversity factor. The CIBSE Guide book A recommends a diversity factor of 0.7 for district heating schemes that have a wide spread and high number of consumers.