basic safety principles - safety relays for machinery

In the scope of machinery applications, Safety-related parts of control systems uses in most cases relays or contactor.

These components have an "non safety" failure mode that is the sticking of contacts or of one contact that prevents others from opening contacts and prevents to place the machine in a safe state, eg a state without energy in order to supress the movements.

The relays used in the  safety machinery applications must comply with design requirements regarding their ability to not contact welding output.These relays are called "Relays with forcibly guided (mechanically linked) contacts" and comply with the requirements of EN 50205 standard.

Other terminologies are also used such as:

  • relays with guided contacts
  • relays with mechanically linked contacts
  • safety relays with non-overlapping contacts.

This European Standard EN 50205 with the following title "Relays with forcibly guided (mechanically linked) contacts " specifies special requirements and tests for elementary relays with forcibly guided contacts, also known as mechanically linked contacts. These special requirements apply in addition to the general requirements of EN 61810-1, EN 61810-5, EN 116000-3 and EN 60255-23.

Notes states that:

  • EN 50205 is not applicable to electromechanical control circuit devices as described in other standards such as the following standards: IEC / EN 60947-5-1.
  • Indeed, this standard provides a definition for the relay in Annex L of the standard (EN 60947-5-1: 2004). This type of relay is: mechanically linked contact elements: combination of n element (s ) to make contact and m member (s) contact opening designed so that they can simultaneously be in the closed position, under the conditions defined in L.8.4

The main requirement of this standard that is  a "principle of safe design" is that an active contact (working contact) and a non-active contact (non-working contact) can be simultaneously closed. This imposes rules of construction of the safety relays.

Technical solutions that prevent closing of the the contacts of a same nature (working contacts or non-working contacts) when one of the complementarycontacts is welded  are:

  • mobile and fixed blades (the elements that carry contacts and transmit the current) which have a stiffness so that they are able for an elastic deformation which is relatively low compared to their normal movement
  • a mechanical device is placed between the movable parts and the non-movable part closer to the contacts in order to maintain an equal spacing of the contacts of the same type (working contact or non-working contact)

The Basic safety principles and Well-tried safety principles - which is also a principle of design and manufacture is as follows:

With respect to the diagram below, this means that a relay with mechanically linked contact must have at least one opening contact and at least one  closing contact and that this closing contact includes mechanical measures that the opening contacts and the closing contacts ("non-working" contact and "working" contact) to be simultaneously closed. 

  • The relay has "working" contact) and "non-working" contact.
  • In normal operation, the status of these contacts is complementary and reversed.

In this diagram, two distances are defined:

  • d1 = nominal distance between the two movable parts of the two complementary contact elements,
  • d2 = maximum distance between two stationary parts of two additional contact elements

In normal operation, the distances d1 and d2 are the same

  • In case of default, the defaulting working contact (Contact welded) prevents the non working contact to change of state. Thus by reading the state of the relay contacts, it is possible to know if a contact fails.

In this case two new distances are defined:

  • d3 = maximum distance between the moving parts of two complementary elements of contacts
  • d4 = residual distance between the two parts of a set of contacts in the welding of a complementary contact.

According to this standard: for a relay or contactor to be considered mechanically linked contact, in the case of  welding of contacts two conditions must be met:

  • first condition: complementary contacts must remain open
  • second condition: the distance between the contacts (d4) must be greater than 0.5 mm for simple contacts "at openning mode"
  • second condition: the distance between the contacts (d4) must be greater than 0.3 mm for the double contacts  "at openning mode" through the endurance characteristics of the relay specified by the manufacturer

In addition, this type of relay must also have a certain level of reliability, since the standard specifies a minimum mechanical endurance of 107 cycles.

This standard also defines two classes:

  • Class A were all contacts are mechanically linked
  • Class B were non mechanically linked contacts can be mixed on a relay with mechanically linked contacts .

The marking on the relay is defined as follows: