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 Machinery Safety 
 Using Safety Relays 


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Machinery Safety and Safety Equipment

 
Machinery safety is the concept of designing machinery and constructing structures and mechanisms in a manner that prevents disasters even if a machine breaks down or when an operator makes a mistake. This could also be described as an approach that enables machines to be used safely even in the event of mechanical troubles or human error.
Reducing risks under this concept of machinery safety requires the use of safety devices.

  1. The term "safety equipment" refers to equipment designed to enable safe operation and maintenance of machinery equipment at factory facilities, chemical plants, and other locations.
  2. The purpose of safety equipment is to ensure the safety of workers in the event of machine failure or malfunction. As such, safety equipment differs from ordinary products and requires specific safety functions.
  3. The following are used in safety-related electrical control systems: Safety switches, Emergency stop push buttons, Safety light curtains, Safety relay modules, and Safety relays, etc.

This document introduces safety standards and specific applications with a focus on "safety relays," which are required to ensure machinery safety.

Product examples of safety equipment

product_examples_of_safety_equipment

International Safety Standards

System of international standards

International standards for machinery safety are created by the International Electrotechnical Commission (IEC), which conducts international standardization primarily in the fields of electric/electronic technologies, and the International Organization for Standardization (ISO), which conducts international standardization including fields other than electric/electronic technologies (machinery, management, etc.) . In particular, the ISO/IEC Guide 51 is published to indicate the concepts common to safety standards issued by each organization.

System of international standards

Design Considerations for Safety Systems

Safety standards and safety relays

EN/ISO 13849-1: Performance Level PL
Performance Level
(PL)		 Probability of Dangerous
Failure per Hour
(PFHd) 1/h
a 10-5 < PFH < 10-4
b 3 x 10-6 < PFH < 10-5
c 10-6 < PFH < 3 x 10-6
d 10-7 < PFH < 10-6
e 10-8 < PFH < 10-7

Parts that execute safety functions for machinery are called "Safety-related parts of control system", and the classification used to regulate the capabilities of these safety-related parts is called "Performance Level (PL)".

EN/IEC 62061: Safety Integrity Level SIL
Safety Integrity Level
(SIL)		 Probability of
Dangerous Failure
per Hour
No special safety
requirement		 10-5 < PFH < 10-4
1 3 x 10-6 < PFH < 10-5
1 10-6 < PFH < 3 x 10-6
2 10-7 < PFH < 10-6
3 10-8 < PFH < 10-7

The term "functional safety" refers to achieving a state in which there are no unacceptable risks through the implementation of functions that ensure safety.
In addition, functional safety standards specify effective methods for implementing functional safety. In particular, SIL is determined based on probabilistic indicators
(the average probability of dangerous function failure and the average frequency of dangerous failures).

▶ Safety function failure probability is determined by the combination of the parts with safety functions! 

Safety relays with forcibly guided structures themselves are originally neither ISO13849-1 (PL) nor IEC62061 (SIL) compliant. For application producers who require a safety standard (e.g., PLC/safety light curtains) , B10/B10d values can be provided to prove the failure probability of safety functions.

Safety_standards_and_safety_relays
  • B10 refers to the average number of cycles before 10% of parts or systems failure (including both safe and dangerous failures).
     
  • B10d refers to the average number of cycles before 10% of parts or systems failure in a dangerous side.

    Note) When a B10 value is available, the B10d value may in some cases be calculated by doubling the B10 value.

Example SF-Y relays:
Load B10d
AC1 230 V 6 A 2 M
AC15 230 V 5 A 35 k
DC1 24 V 6 A 2 M
DC13 24 V 4 A 500 k
DC13 24 V 2 A 1.6 M

 

Why are safety relays necessary?

There is no obligation to install safety relays in safety equipment. However, installing safety relays facilitates designing safety equipment, simplifies the approval process, and ensures that safety equipment is deployed on time.

 

Item General relay Safety relay
(Forcibly guided contact structure)
Diagnostic coverage & Average
DC: Diagnostic Coverage & DC avg		 From 0%
Board designers need to increase the DC value
using circuits to reach the required PL and SIL.		 99%
Enables board designers to reach the
required PL and SIL more easily.
Mean cycles to failure
(B10/B10d)		 Very little information
Even if there is only B10.		 The measurement method of B10d is
compliant with DIN EN 61810-2-1.
Coil failure detection with
safe feedback contacts		 Board designers need to take
appropriate measures.		 Compatible with relay structure
Failure detection with safe feedback
contacts (contact welding)		 Not compatible Compatible
Other contact gaps at the time
of contact welding		 Failures due to contact welding cannot be
detected due to insufficient contact gap.		 Securing of contact gap min. 0.5 mm ensures
reliable detection of contact welding and other factors.
Also, since it is compliant with IEC61810-3,
it is ideal for safety circuit design.
Price level Low price
However, other parts and systems are
needed in order to achieve machinery
safety, which results in additional costs.		 Slightly higher price
(Due to additional design to ensure compatibility
with forcibly guided structures, such as
multiple contacts and contact GAP securing, etc.)

Differences between safety relays and general relays

  Safety relay General relay
Structure Safety relay General relay
Features
(1)
  • Safety relays have two types of contacts:
    N.O. contacts (normally open) and
    N.C. contacts (normally closed).
  • Although these two types of contacts are separated
    by a wall and do not interfere with each other,
    they are mechanically connected by a guide.

The following is not guaranteed.

  • Although these two types of contacts are separated
    by a wall and do not interfere with each other,
    they are mechanically connected by a guide.
Features
(2)		 Safety relay General relay

Product Lineup

Rich lineup

lineup

Specifications

Product name SF-M SF Slim SF-Y SF SF Double contact
Structure
SF-M
SFS
SFY
SF
SF-DO
Contact
arrangement		 1 Form A 1 Form B 4 poles:
2 Form A 2 Form B,
3 Form A 1 Form B
 
6 poles:
4 Form A 2 Form B,
5 Form A 1 Form B,
3 Form A 3 Form B		 4 poles:
2 Form A 2 Form B,
3 Form A 1 Form B
 
6 poles:
4 Form A 2 Form B,
5 Form A 1 Form B		 3 Form A 1 Form B 2 Form A 2 Form B,
4 Form A 4 Form B
Contact
rating		 N.C. : 4 A 250 V AC 30 V DC
N.O. : 6 A 250 V AC 30 V DC		 6 A 250 V AC 30 V DC 6 A 250 V AC 30 V DC 6 A 250 V AC 30 V DC 6 A 250 V AC 30 V DC
Min. switching
load
(reference value)		 1 mA 10 V DC 1 mA 5 V DC 10 mA 10 V DC 100 mA 5 V DC 100 mA 5 V DC
Rated operating
power		 Operating: 270 mW
Holding: 100 mW		 4 poles: 360 mW
6 poles: 500 mW		 670 mW 500 mW 500 mW
Rated coil
voltage		 3, 5, 12, 16, 18, 21, 24 V DC 12, 24, 48 V DC 5, 12, 16, 18, 21, 24 VDC 5, 12, 24, 48, 60 V DC 5, 12, 24, 48, 60 V DC
Ambient
temperature		 -40°C to +85°C -40°C to +85°C -40°C to +70°C -40°C to +70°C -40°C to +70°C
Safety
standard		 UL/C-UL, TÜV UL/C-UL, TÜV, Korean S, CQC UL/C-UL, TÜV UL/C-UL, TÜV UL/C-UL, TÜV
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Application Introduction

 Self-holding Circuit during Emergency Stop 

■ Typical example Production equipment: Emergency stop circuit

Self-holding Circuit during Emergency Stop

 Detect Abnormalities and Stop the System 

■ Typical example Production equipment: Abnormality detection and stop circuit when contact welding occurs.

Detect Abnormalities and Stop the System

 Ensuring Safety through Sequence Control 

■ Typical example Production equipment: Sequence circuit

Ensuring Safety through Sequence Control

 Coordinated ON / OFF of Control Circuit that must not Operate Simultaneously 

■ Typical example Production equipment: Switching circuit

Coordinated ON / OFF of Control Circuit that must not Operate Simultaneously

 Safety relays only → Enable realization of Equivalent Safety Circuits using Safety Relays together with Electronic Circuit 

■ Typical example Production equipment: Abnormality detection circuit

Safety relays only → Enable realization of Equivalent Safety Circuits using Safety Relays together with Electronic Circuit

 Safety Circuit Construction using Safety Relays and Examples of Functions 

■ Typical example Production equipment: Safety door

Safety Circuit Construction using Safety Relays and Examples of Functions