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M & R ELECTRICAL WHOLESALING

Knowledgebase Article

Machine Safety Standards

 

Introduction

All machines designed in Australia need to conform to the machinery safety standard AS4024. Many engineers know about this standard and use it when designing large and complex machines. However, in many parts of Australia it is common for smaller machines to be designed by electricians, fitters and boilermakers. While they are perfectly capable of doing this, they are often unaware of the machinery safety standard and how it directly relates to them. This document discusses some of the important aspects of this standard, and has a particular focus on the electrical design.

 

If as part of your job you design, assist with design or connect power to machines of any size (such as conveyors, pumps etc) and are unaware of this standard you should read on.

 

Liability

Remember that you are liable if an accident occurs on a machine that was not designed properly. In Australia you can no longer use the legal defence of being ignorant of a standard's existence. The fines can be severe, and in many states can involve a period of incarceration.

 

AS4024

This standard outlines the aspects that any machine should have to be considered safe. It is one of the largest standards and covers many aspects, such as machine guarding, signage, pollution controls and even operator training. These standards apply to all machines made or used in Australia, from a simple irrigation pump to a multimillion dollar iron ore reclaimer.

 

The newest version of this standard was introduced in 2006. With the update some important changes were made, particularly in connection with the control circuit. These changes were made to bring us more in line with European standards.

 

Control Circuit

One of the most important parts of the machine design is the control circuit. The way the control circuit is designed determines the safety of the machine. For example, are the limit switches wired normally open or normally closed? There are strict rules to be followed to determine what level of safety is required, and in turn how the control circuit is designed.

 

Depending on the hazards that the machine presents and how frequently the users are exposed to these hazards determines what safety category the control system needs to conform to. There are 5 safety categories, which are listed below.



 

Safety categories for control systems conforming to per. AS 4024.1-2006

Categories
Summary of requirements
System behaviour
Principles to
achieve safety
B
The safety related parts of the control system and/or its
protection devices, as well as their components, shall
be designed, constructed, selected, assembled and
combined in accordance with relevant standards, so
that they can withstand the expected influence.
– The occurrence of a fault can lead to a loss of the safety function.
– Some faults remain undetected.
Mainly characterised
by selection of
components
1
Requirements of B shall apply.
Well-tried components and well-tried safety principles
shall be used.
The occurrence of a fault can lead to
loss of the safety function, but the
probability of occurrence is lower than
for category B.
2
Requirements of B and the use of well-tried safety
principles shall apply.
Safety function shall be checked at suitable intervals
by the machine control system.
– The occurrence of a fault can lead to
loss of the safety function between
the checks.
– The loss of safety function is
detected.
Mainly characterised
by structure

The correct selection
of safety components
and principles
3
Requirements of B and the use of well-tried safety
principles shall apply.

Safety-related parts shall be designed so that:
a) A single fault in any of these parts does not lead to
loss of the safety function; and
b) Whenever reasonably practicable the single fault is
detected.
– When a single fault occurs, the safety function is always performed.
– Some but no all faults will be detected.
– Accumulation of undetected faults can lead to loss of the safety function.
4
Requirements of B and the use of well-tried safety
principles shall apply.

Safety-related parts shall be designed so that:
a) A single fault in any of these parts does not lead to
loss of the safety function; and
b) The single fault is detected at or before the next
demand upon the safety function. If this is not possible,
then an accumulation of faults shall not lead to loss of
the safety function.
– When the faults occur the safety
function is always performed.
– The faults will be detected in time to prevent loss of the safety function.

 

A formalised risk assessment should be done to determine what level of safety is required for the machine.

 

 

Guarding

Machine guarding is very important in managing the hazards of a machine, but unfortunately it is very often either not designed properly or completely left out. Before connecting power to any machine you should make sure it has adequate guarding so that a person cannot be injured while using the machine. There are rules about the minimum spacings in guarding, to ensure that children cannot place their hands through the guarding. It is important to ensure these rules are met.

 

In cases where guarding needs to be regularly removed for maintenance reasons, you should consider installing a limit switch that detects when a guard is removed. By integrating this into the control circuit you can ensure that the machine is stopped if the guard is removed.

 

Sirens

In some cases, a pre-start siren is required on the machine to alert people it is about to start. Most conveyors fall under this category.

 

Emergency Stop Buttons

Every machine has to have at least one emergency stop button. More buttons should be added if the machine is large. How the emergency stop button is wired into the control circuit depends upon the safety category.

 

Conveyors fall under a separate (but related) standard that requires them to have an emergency stop button at each end of the machine, plus tripwires along both sides and along its length.

 

Isolators

Every machine needs a way to be isolated. This isolator needs to completely remove power from the machine and be lockable. Having the isolator simply issue a stop command to the control circuit is not acceptable. Using an emergency stop as an isolator is not acceptable.

 

Signage

Adequate signage should be placed on the machine to keep it safe. All buttons should be labelled, including emergency stop buttons. All isolators need to be clearly marked.

 

Summary

Adhering to the machinery safety standards is a very complex part of a machine's design. However, it is also a very important part of the design that cannot be forgotten about. You cannot rely solely on following the rules of AS3000 to wire up a machine.