Marechal Decontactor Plugs & Sockets - The Advantages Of Butt Contacts

By Pamela Kinnell on 13th February, 2014

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Marechal Decontactor Plugs & Sockets - The Advantages Of Butt Contacts


Marechal Plugs & Sockets 

Advantages Of Butt Contacts

The Marechal decontactor withstands temporary overloads due to starting or frequent restarting of motors, pumps or fans (that generate the longest overload) without any damage.

Conventional brass pin and contact-tubes have short time constants and reach their thermal equilibrium very quickly.

They are unable to withstand such overloads without heavily oxidising the contacts, welding and becoming unusable.

The ability of silver-nickel butt contacts to withstand high in-rush currents and overloads makes them particularly suitable to connect motors, with invaluable advantages:

  • A plug-in motor can be quickly replaced, saving costly downtime
  • A plug-in motor does not require a qualified electrician and PPE
  • A plug-in motor eliminates the need for an isolating switch next to the motor as it can be visibly isolated from the mains by non-technical personnel, by withdrawing the plug from the socket-outlet.

Most Marechal decontactors have a utilisation category of AC-23 A and can be used for the permanent connection and the occasional switching of electrical motors.

Other butt-contact Marechal plugs and socket-outlets that do not have a utilisation category of AC-23 A have nevertheless the same ability to withstand temporary overloads and can also be used to connect motors, under the provision that they are electrically interlocked with the motor starter or a contactor, or mechanically locked, to prevent disconnection under load.

Marechal Industrial Plugs & Sockets

Short-Circuits : Withstand & Close-On Tests

Two cases must be considered:

  1. Short-circuits occurring when the contacts are closed
  2. Short-circuits occurring when a plug is introduced into a socket-outlet, because the cable or the appliance is defective

The latter is likely to have disastrous consequences because of the arcs that strike when the contacts of the plug are about to mate with those of the socket-outlet.

If the mating is gradual, as with pins and contact-tubes, the brass tubes disintegrate, producing metallic oxide gases.

A phase-to-phase or phase-to-ground short- circuit results inside the socket-outlet which may explode.

As the connection is never completely made, arc resistance and flowing currents may not be large enough to trip the upwards protection in time.

The butt contact closes when the two solid metal parts touch - the current flows and the protection trips. Circuit-breakers are the best illustration of the suitability of this design.

Third-party tests have been carried out according to North American standards to check the behaviour of the decontactor under short-circuit condition, in conditions of unfavourable circuit protection, using time fuses rated at 2.5 to 4 times the rating of the socket-outlet.

The decontactor passed both the withstand, and the close-on (make) test for conditional short-circuits of 10 000 to 200 000 A.

To the knowledge of Marechal, the decontactor is the only plug and socket-outlet in the world capable of offering such a safety when closing on a fault.


Marechal Decontactors - Ejection & Contact Springs



IEC/EN 60309-1 standard (plugs and socket-outlets for industrial purposes) sets, in clauses 20 and 21, a minimum load-breaking capacity.

These tests are followed by a temperature rise test that must not exceed 50 K and a dielectric strength test.

According to the standard, a 125 A plug and socket-outlet is expected to perform 250 on-load operations to meet standard requirements.

Performance of the decontactor is well above the standard and more in-line with market requirements.

See Also : Marechal Ex Multipin Connectors For Hazardous Area Power, Signal & Data Transmission

Marechal Decontactor Plug and Socket Outlets


Some Relevant Comments From LinkedIn Discussion ..

"Re Switching: Most Marechal decontactors have a utilisation category of AC 23A and can be used for the permanent connection and occasional switching of electric motors. (EN 60947-3).

EN 60309-1 sets out minimum load breaking capacity for socket - outlets. For example at 125A, 250 operations are required at a power factor of 0.7.

The Marechal 125A decontactor handles 3000 such operations at 0.7pf

UL listed switch rated decontactors are listed with short circuit ratings of at least 65kA achieved at 600Vac and 0 .15 pf.

Marechal decontactors use silver-nickel spring-loaded butt contacts which eject the plug to a parked position when the socket release latch is pressed.

They are not pin & sleeve devices. This contact design provides for the electrical performances achieved." Stephen Thackray, UK Country Manager (Marechal Electric). 

"I now see where several colleagues are coming from. The Marechal plug appears to have been designed for use in Zone 1 and Zone 2 hazardous area locations.

The principle used goes way back at least 30 years to my knowledge. Intrinsically safe light sockets used to be made using the same principles. ie when the bulb was pushed into the socket it made contact with pins that were spring loaded and "dead".

As the bulb was pushed in, the pins were also pushed in to make contact with the supply pins inside an Ex rated box (the body of the bulb holder).

In my view, the 63Amp plugs described are intended to be used in Zone 1 and Zone 2 atmospheres (ie if current is caused to flow accidentally, the arc produced is inside the Ex box).

I do not imagine that they were designed to be used to pull the plug on live equipment since no matter where the disconnection arc is produced, the connections are still damaged. There is a (not very) subtle diference." Roy Nightingale, JP. CFIOSH. I.Eng. FIET. Chartered S&H Practitioner, Consultant Safety Engineer.

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