There are statutes and regulations that deal with safety requirements of machines; compliance with these is of course obligatory; in this section of this article some particular aspects are considered.
In England the Health and Safety Executive publish numerous documents regarding safety many of these are freely available from their website and are an excellent reference and source of guidance. Although all come with disclaimers so it is still your responsibility to demonstrate compliance with the legal requirements.
Also don’t ask their advice as they will only point to the relevant publications or statutory instruments (the establishments name for laws) and will never commit themselves or make a decision (graduate from university then years of civil service training), however if there is an accident you may be assured of their intimate attention to your person (they will be all over you like a rash).
The safety of persons in the vicinity of a motor and the plant or equipment that it drives or those operating or maintaining it are paramount and must be the primary consideration of the system designer.
More and more motors are started and stopped under remote or automatic control, in these instances there is no visual check that it is safe to start a motor, therefore systems and procedures must be put in place to ensure safety.
Guards that prevent access to a machine and it's moving parts are in general the first line of safety for those in it’s vicinity. It is primarily the machine designer’s responsibility to provide the guards for a machine. Where guards are capable of being opened without the use of tools then switches that act in the same manner as emergency stop devices must be incorporated, it is essential that the operation of these switches cannot be defeated by wedging or tying up.
Emergency stop devices should be located adjacent to the motor and around the driven machine that over-ride all other controls to remove power to the motor to stop it. These devices must be clearly marked, must not be shrouded and should have a head which is red in colour is at least 38mm in diameter and is mounted on a box with a yellow cover. All emergency stop devices must when operated be of the type that require a manual reset, often referred to as stayput, and the control circuit so arranged that the motor cannot be started if an emergency stop device has been operated and has not been reset.
The release of an emergency stop button or any other stop button must not under any circumstances cause a motor to either start or restart, it should only permit the motor to be started or restarted and that start or restart should be by the initiation of a start button or auto run enable button.
Where the plant being driven is a conveyor belt; a pre-tensioned trip wire system configured to stop the motor should the trip wire go slack or be pulled, the switch once tripped must be reset manually before the motor can be re-started.
To enable the motor to be started and stopped locally at the motor by the maintenance engineers, each motor should have adjacent to it a local control station (LCS) that incorporates a keyswitch, a start button or for reversing and multi-speed motors buttons, and an emergency stop button. The location of the local control station is of importance, it must be located on the escape route from the motor it controls yet adjacent to the motor. The keyswitch must have two positions local and remote, when local is selected, control must be at the local control station, the control circuitry must prevent control from any other place, only the emergency stop devices, motor protection devices and inhibit interlocks should be in circuit. Permissive control to other motors on the same system must be inhibited. When remote is selected the start button(s) become(s) ineffective and control is passed to the next higher level control point.
The key that operates the keyswitch should be trapped when local is selected and free when remote is selected. Electricians and fitters hate losing their keys, which they tend to treat as badges of office and are therefore prized possessions, which they rarely if ever lend to others, so it will be an extremely rare occurrence for them to forget to set the local control station back to remote when they have finished their task as it is the only way of retrieving their key!
The practice of fitting start and stop buttons in the MCC or in a remote starter in lieu of a local control station, although being much cheaper to implement, is not satisfactory as the control will likely be remote from the motors that are controlled from that MCC or starter and some motors may even be out of sight of the MCC or starter.
A properly performed risk assessment will show that this is not conducive to either safety or safe operating procedures and that there is an increase of the risk of dangerous occurrences.
The provision of local isolation is a requirement of some clients; the strange thing is that it is common to see local isolators on small low voltage motors but extremely rare to see them on either large motors or medium voltage motors or high voltage motors. A point that must be made is how is the ‘means of local isolation’ maintained, there will be no local isolation for the local isolation! Far better to use the fused switch in the starter to isolate the motor, it also has the benefit of isolating the control circuits, providing that the labelling is properly done and is unambiguous.
The requirements of regulation 12 – Means for cutting off the supply and for isolation: of the Electricity at Work Act 1989 (SI No. 1989-635); in this regulation cutting off the supply means switching off and is different to isolation; are met if the local control station described within this article is equipped and wired as described and shown on the wiring diagrams; and the starter is equipped with the lockable fused switch described on the wiring diagrams; and the starter and motor are clearly marked with the motor ident and reference.
Where local isolation is a requirement the isolator must be co-located with the motor, isolate all the motor supply cables, which can require as many as twelve main contacts. The isolator must also be fitted with auxiliary contacts, one of the auxiliary contacts should open before the main contacts open when switching ‘off’, that auxiliary contact should be configured to de-energise the starter contactors and close after the main contacts close when switching ‘on’ to enable the starter contactors to energise. This auxiliary contact is used to ensure that the isolator main contacts operate on no load.
The isolator must also be capable of being padlocked in the off position; the components of the isolator used to effect the padlocking should be of a robust design that prevents defeat of the padlocking by vandalism.
Local isolation should not be considered as an alternative to emergency stop buttons as the physical operation of the two devices is different, it takes longer and requires a more complex manual operation to switch an isolator off than it does to simply strike an emergency stop button.
Local isolation, where required, should be considered as complementary to the emergency stop button(s) not an alternative.
A warning that the motor is about to start should be incorporated into the starting system, this is to allow a person working on or engaged in some other activity on or near a motor or the driven plant time to initiate an emergency stop device. The fact that a person has to activate an emergency stop device when the starting sequence is initiated by the start alarms is a procedural beach as the motor should have previously been isolated.
That breach of procedure should be treated with the seriousness it merits.
The start alarm should be both audible and visual; it should take the form of two tone warblers and amber flashing beacons that sound and flash for at least ten seconds before the motor starts. These should be located so that a person near the machine can see and hear the alarms.
The audible and visual alarms must be unique to the plant start alarms; there must be no confusion with other alarms such as fire alarms, evacuation alarms, plant failure alarms etc.
To enable the motor and it's starter to be isolated for maintenance the starter should be equipped with a fused switch or circuit breaker which can be padlocked in the off position.
To ensure that isolation is carried out for the correct motor the labelling of the motor, it’s supply cables and it's starter must be clear and unambiguous. As well as plant titles reference idents should be used. All the cables associated with a motor should be marked at their points of termination. All documents and manuals must have the same references.
The fused switch or circuit breaker must not only isolate the main power supply to the starter but also the control, alarm and indication circuits within the starter. Where a motor is supplied with a voltage above low voltage the starter must be equipped with facilities to earth the motor and it’s supply cables.
Many motors have their starters grouped together in motor control centres (MCCs). Where this is the case not only should the starter have a plant title and reference identifier, it's terminal chamber should also have on it's cover and inside the terminal chamber itself the plant title and reference identifier. By placing a label with the plant title and reference identifier in the non-removable part of the terminal chamber it will help to prevent the danger that can arise when terminal chamber covers are replaced on the wrong chamber.
Many companies use asset registers to track their equipment and for the issue of planned maintenance work sheets. Where this is the case the asset register must contain a column for the insertion of the plant title and reference identifier, this is to ensure proper cross referencing. Any planned maintenance worksheets must also contain the plant references.