Here we must consider the different forms of electrical supply, and how the operative is pro- tected from the potential dangers associated with them. For example:
● Tools using a mains electricity supply. ● Tools protected by Double Insulation. ● Tools using a 110volts supply
● Tools connected via a Residual Current
Device (RCDs)
● Tools run off Portable Electricity Generators ● Safe use of electrically operated hand power
tools.
6.2.1 Mains supply
Figure 6.2 shows how an electric drill or similar machine is protected by an earth line (wire). The principle is as follows. If the electrical appliance has a low-resistance connection to earth, then a return path is available for the current if a fault occurs that causes the appliance’s casing to become live. This low-resistance path allows a high current to flow which causes the fuse (pro- viding it is correctly rated) to burn out (blow), thus stopping the flow of current and rendering the machine safe.
No matter how effective this system might seem, there is always the possibility that it will not work when it is most needed. For example, the system will fail to work if the earth wire has:
● not been connected to the plug socket, or
has become defective en route or at source,
● become disconnected from the machine, ● become disconnected from the plug, ● been damaged within the flexible cable to
the machine, or the extension lead.
If the earthing system fails to work, possibly for one of the above reasons, it could result in the operator’s body being used as an electricity escape route to earth – the results of which could prove fatal.
Fortunately, nearly all portable power tools produced are now double-insulated.
6.2.2 Double insulation
Figure 6.3 shows how a double barrier is formed around all those components capable of con- ducting an electrical current. This is achieved by using a strong non-conductive material for the body and/or isolating any metal parts with a non-conductive inner lining, thus eliminating the need for an earth wire.
Portable power tools which are double insu-
Contacts Cartridge fuse Holder clip Live pin Connecting block Live wire (brown) (Brown sheath)
Section through a cartridge fuse Cartridge fuse (a) Cartridge fuse
(b) Three pin 13 amp plug
(c) Three pin connection to a power tool
(d) Earth wire symbol Double pole switch
Earth Cartridge fuse Wire connection to a 13 A three pin plug Live wire brown sheathing Live wire Neutral wire blue sheathing Earth wire green and yellow
banded sheathing Pin geometry 240 V 13A Neutral pin Plug body 3 core cable Live pin Earth pin Plastic or fibre casing High resistance
fuse wire will burn out
L
Holder clip
lated bear the symbol of a square in a square and, before these tools can legally be used in industry, dispensing with their earth wire, they must comply with the Electricity at Work Regulations 1989 by being approved by the British Standards Institution and bearing BSI’s ‘Kitemark’ BS 2745 and 2769 on the casing.
Double-insulated tools are undoubtedly safer than single-insulated tools (normal earthed sys- tems) but, unless they are powered from a low voltage supply (110 V), there is still considerable danger from the current-carrying cable.
6.2.3 110v Power supply
This is the specified voltage for power tools used on a building site figure 6.4 shows how the sup- ply voltage of 240 volts is reduced to 110 volts, with a centrally tapped earth, so that, if a break- down in insulation does occur, the operative should only receive a shock from 55 volts. 6.2.4 Portable electrical generators
Were a mains supply is not available, one alter- native is use a portable generator powered by either a petrol or diesel engine. A typical example is shown in figure 6.6. These are available with different power outputs. Figure 6.5 shows the electric plug/socket geometry.
Generators must be sited outdoors (in the open air) – exhaust fumes are toxic. It must be installed according to BS 7375 by a competent electrician and must be earthed.
6.2.5 Residual current device (RCDs) (fig 6.7)
These are devices which, in the event of an elec- trical fault occurring where current flows to earth. They work by detecting an imbalance in the flow of electricity between the live and neu- tral wires in the circuit such as would occur if Earthing, insulation & electrical safety 169
External (body) insulation
Double insulation symbol British standard symbol Double pole switch Internal insulation Neutral wire
Fuse Live wire
Fig 6.3 Double insulated tools and their symbols
Keyway geometry 240 V plug Blue Keyway geometry 110 V plug Yellow
Fig 6.5 Keyway geometry
Transformer housing earthed
CPC Circuit Protective Conductor
110 V Power tool
110 V Transformer (identification colour yellow) Cartridge
fuse Socket and plug
B5 4343
there was a leakage to earth, in which case the double pole switch would disconnect the supply almost instantaneously (fig 6.7a). They do, how- ever, depend on the proper functioning of their electrical and mechanical components, it is essential therefore, that they are tested at regular intervals (before each work period) – test but- tons are provided to enable this to be carried out.
RCDs are available in several forms;
● One which is permanently combined with
the plug and directly wired to the tool.
● Built into the mains consumer unit. ● A socket outlet in a ring main system (fig
6.7b).
● An adapter fitted between plug on the tool
lead and the socket (should not be used – see below).
Note: RCDs adapters (fig 6.7c) should not be used
as there is always the risk that they may accidentally be forgotten or left out of the system.
6.2.6 General safety
Before a portable power tool is used, the opera- tor must be confident that all necessary steps have been taken to ensure both his/her safety and that of any persons within close proximity of the operation to be carried out.
The precautions listed below should always be followed:
1 Never use a portable power tool until a competent person has instructed you in its use.
2 Only use a portable power tool after authorised approval (the tool in question may have been withdrawn from use or some reason of safety).
3 The manufacturer’s handbook of
instruction for the tool in question should be read and understood before use. 4 Always wear sensibly fitting clothes – avoid
loose cuffs, ties, and clothes which are torn etc.
5 Wear eye protection where there is a risk of propelled debris or fume, dust or mist which may impair or damage your eyesight. 6 The correct type of Dust masks should be
worn where the operative’s health may be at risk.
7 Ear protection should be worn by all operatives who are likely to be subjected to Fig 6.6 Portable generator
Supply Load Test button Test resistor L N N L Trip coil Double pole switch Fault detector coil (a) (b) Test button Indicator light Reset button RCD Socket Outlet RCD Adaptor
Fig 6.7 Residual current device (RCD)
a noise at 85dB(A) and above, for example if you have to shout to make yourself heard by someone 2 metres away.
Examples of some portable powered hand tools where approved ear protection should be worn are: – Impact (percussion) drills, circular saws, planers, routers, and cartridge operated tools. Under the Noise at Work Regulations 1989 employers are required to reduce the risk of hearing damage.
8 Guards, where fitted, must always be used. 9 Never use blunt or damaged cutters. 10 Keep flexible cables away from the work-
piece, cutters, and sharp edges and also from trailing on the floor.
11 Before changing bits, abrasive sheets or making any adjustments, always disconnect the tool from the electric supply (remove the plug from its socket).
12 If a tool is damaged or found to be defective, return it to the stores or to the person responsible for it. Ensure that it is correctly labelled regarding the extent of its damage or defect.
13 If injury should occur – no matter how minor – first aid must be applied immediately to avoid the risk of further complications. The incident should then be reported to the person responsible for safety. Also see appendix 1.