Plugged Versus Soldered Safety Relays

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Plugged or soldered?

The type of assembly that should be preferred for safety relays.

Absolute safety must be warranted for relays with positive-guided contacts to EN 50205 for use in circuits serving for the safety and the protection of persons and facilities against hazardous motions. This requirement is met for the relay inside a safety controller but frequently not for the mechanical and electrical contact between the connection of the safety relay and the terminals of the relay socket.

Stefan Illien*

Manufacturers of safety facilities assume from the start a high degree of safety and reliability within the permissible application range of the selected relay. But the qualities and strengths of a safety relay are only optimally utilized when mounted and soldered direct on the printed circuit board (PCB).

Certain users in machinery and plant construction as well as in the elevator branch prefer to use relay sockets for service reasons. The simplicity of swapping relays at first glance may present considerable advantages, but when looking in detail at the specialties of this type of relay connection the disadvantages predominate: the mechanical as well as the electrical connection between the commonly filigreed pins of the relay and the socket clamps frequently is insufficient. Safety relays that are designed for PCB assembly principally do not feature suitable connections required for plug sockets.

For economical reasons as a rule only half-hearted clamp part solutions are applied for these accessories. The advantage of quickly exchanging a defective relay is soon offset by other functional disadvantages.

ELESTA relays GmbH has analyzed this issue already years ago, and in the end decided against offering a socket for their relays. The reasons are the operating conditions that prevail in machinery and elevator controls, which are often underestimated or even ignored by developers of safety-oriented controllers. External influences act on the components, and which can disturb or even defeat the demanded relays functions. At that, the influence of shock and vibration must be closely observed. In addition, great differences in temperature as well as humidity over time are not improving factors for the relay to socket connection.

The clamp is a weak point of the plug socket.

Frequently, the clamp of the customary socket is problematic already from a design point of view. Inadequate clamping force, 2-point contact area for each relay pin, base metal surface incompatibilities (oxidation) and material fatigue after a few times plugging reduces the functionality drastically. The relay and the socket must also be capable of switching and continuously conducting a small, medium and large burden of several amps. Fatigue and oxidizing of the clamping points lead to unreliable contacting. The contact resistance increases progressively and the probability of contact faults grows rapidly. Troubleshooting of these faults is difficult and time consuming.

With a burden of several amps this can lead to a dramatic temperature rise that finally ends in welding and burnout of the contact points. Such failures force a machine or plant to a halt and require extensive repairs. At that the damage is not restricted to the relay alone. Simultaneously, the socket must be replaced as well, and in worst case the whole PCB. A look at all possible weak points of a socket, finally reveals how critical it is to consider the use of such accessories.

The optimal use of the relay qualities, inclusive a long service life, is only given if the relay is soldered to the board. In the long run this type of assembly absolutely pays off even though in view of exchangeability it may not make sense at first glance. In a flash the immediate additional costs of a different mounting type can be exceeded by disturbances and failures of machines and plants.

Today, all renowned manufacturers in the field of connection technology offer very universal and sophisticated screw, plug and solder connection solutions. Simple plugs, multiple group connections and various mounting features enable solutions for almost every requirement. Depending on what the need may be, companies like Wago, Phoenix Contacts, Wieland or Weidmüller offer versatile connection elements for relay boards. An ordinary combination consisting of print relay sockets never approaches the quality and reliability of these connection techniques.

Plug and socket fit optimally and ensure perfect contact. This provides for faultless switching of various circuits from the low power range to heavy burdens, and ensures an utmost measure of safety and reliability even after years of service. When defective or at the end of its service life, the PCB with the soldered relay and other components is quickly exchanged. At the manufacturer the faulty component can be replaced by a new part. In addition to that it leaves the controller manufacturer much freedom in the design and arrangement of his assemblies due to the way such connection systems are mounted.

  • Relays with positive-guided contacts for safety applications from ELESTA relays
  • Printed circuit board components from: Phoenix Contact
  • Wago
  • Weidmüller
  • Wieland
  • News, relay technology and application of German relay manufacturers in the ZVEI
  • Circuit boards of all types populated with one or several relays of various types, and in part with circuitry.

Positive guide warranted even in case of a fault.

Unlike conventional relays in the industry line of business, stricter quality measures apply to relays with positive-guided contacts. After all, a safety relay is expected to be highly reliable over its whole service lifetime. The positive guidance of contacts described in the standard EN 50205 must always be maintained even in case of a fault. Normally open und normally closed contacts must never be closed at the same time. The mutual mechanical link prevents this from ever happening even when one contact welds or gets locked. In case of a fault the contact gap must be at least 0.5 mm independent of the service life or the size of the burden.

Stefan Illien
Senior Engineer
Bad Ragaz, 02. December 2003 Elesta relays GmbH

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