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Safety Device Safety Measue Categories

Based on the risk assessment results of a machine, implement safety measures in order to reduce the risks. There are two types of measures to reduce risk, namely, safe design and safety devices, and usually safe design is implemented first.
Before implementing safety measures using a safety device, the degree of risk is determined following the risk assessment flow chart as specified in ISO13849-1(JIS B 9705-1). Then, the category of safety is determined based on the degree of risk and the safety measures are structured with safety equipment and a control circuit accordingly.

Symbol	Description of symbol	Rank	Description of rank
S	Degree of injury	S1	Slight injury (bruises, etc.)
S	Degree of injury	S2	Serious injury (amputation of a limb, death, etc.)
F	Degree of risk exposure	F1	Almost none.
F	Degree of risk exposure	F2	Frequent to always.
P	Degree of risk avoidance	P1	Possible.
P	Degree of risk avoidance	P2	Impossible.

Flow chart for determining safety requirements

Risk level	Selections of category of safety
Risk level	B	1	2	3	4
I
II
III
IV
V

	Safest measures for the risk
	Standard measures for the risk
	Measures which require special means including a protective device
	Incompatible as the measure.

Explanation of Symbols

Following are examples of ranking guidelines.

S：	Injury which requires suspension of operation or which requires further treatment other than first aid is considered as S2 (serious injury).
F：	If it is necessary to access the risk source more than once a day (normal operation, maintenance, refilling, etc.), it is considered as F2 (frequent to always).
P：	If the approach speed of dangerous object is 250 mm/s or above, it is considered as P2 (impossible).

*	Reference standard : ANSI/RIA R15.06

Category Requirements in ISO13849-1

Category

Outline of requirements

Operations of the control system

The safety-related parts of the control system or protective device must be designed, structured, selected, or assembled according to the appropriate standard in order to withstand expected external influences.

The safety feature will be lost upon breakdown.

Must satisfy the requirements of category B.
Use highly reliable parts which are well-examined and comply with the safety principle*.

Although the safety feature will be lost upon breakdown, the probability is lower than category B.

Must satisfy the requirements of category B and comply with the safety principle.
The safety feature must be checked with the control system of the machine at an adequate interval.

The safety feature will be lost if breakdown occurs during the interval.
The loss of the safety feature is detected by regular check.

Must satisfy the requirements of category B and comply with the safety principle. Safety-related parts must be designed according to the following principles.
1.	Safety features shall not be lost by a single failure.
2.	A single failure must be detected as much as possible.

Even if a single failure occurs, the safety feature is always maintained.
Not all failures can be detected.
The safety feature will be lost if undetected failures accumulate.

Must satisfy the requirements of category B and comply with the safety principle. Safety-related parts must be designed according to the following principles.
1.	Safety feature shall not be lost by a single failure.
2.	Able to detect a single failure when the following safety feature is activated or before activated. If it is not possible, the safety feature will be lost even if undetected failures are accumulated.

Even if a single failure occurs, the safety feature is always maintained.
In order that the safety feature will not be lost, the failure is detected before the safety feature is activated.


*	Safety principles are such as; 1. use a fuse for short-circuit protection, 2. moderately flexible parts dimensions, decrease the ratings, 3. design the mechanism in such a way that the circuit opens when a failure occurs, 4. detect a failure at early stage, 5. protective grounding.

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