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Industrial sockets with mechanical locking switch:
what is their purpose?


1) Industrial sockets with mechanical locking switch: what is their purpose?

When you plug in or unplug a plug into its socket, or during maintenance operations, you run the risk of a direct electrical contact, and the operator is subjected to the effect of the electric arc which is produced while establishing and cutting off the current during the live manoeuvre.

Sockets and plugs for industrial applications feature such a geometry that the electric arc between banana plug and socket contact is developed inside a closed chamber, and might cause on the emission to the outside of ionized gases with incandescent particles.

The effect is especially dangerous for the operator if there is a short-circuit downstream the socket.

A short-circuit current is interrupted by protection devices (fuses or automatic switches) within the time set according to the tripping characteristic.

The arc energy that develops thus depends on the type of protection of the socket power supply circuit.

When the short-circuit currents exceed 5 kA, the arc can become very dangerous (violent expulsion of gas with explosive phenomenon).

When you plug in or unplug the plug, the presence of solid particle or dirt determine the path of the electric arc that may occur between the banana plug and the socket contact prior to the galvanic contact, with considerable danger for the operator which, at that moment, is carrying out the connection.

The arc generated during the indicated operations can also cause fire if the socket is located in environments at risk of fire due to the presence of flammable substances, which is very frequent in the industrial sector.

In systems powered by their own transformer station (TN systems), it often happens that the short-circuit current, event at the plug and socket level, well exceed 5 kA, so in order to avoid the risks described above it is advisable to plug in/unplug the plugs with no voltage on the contacts.

The plug sockets with mechanical locking switch (interlocked sockets) guarantee this characteristic.

Domestic standard interlocked socket
Set of industrial sockets
Interlocked socket



2) How is the mechanical lock carried out?

The plugging in/unplugging with no voltage present can be ensured by using a socket with locking device. The images here below illustrate the operating principle of the mechanical lock that prevents the plug from being plugged in or unplugged when the contacts are live.

By plugging in the plug, the disconnect switch command is freed by means of the lever (red), which can be closed

When the switch is closed, the plug cannot be removed from the socket thanks to the mechanical lock that acts on the projecting tip of the plug (red part)



3) Socket disconnect switch: electrical characteristics

In order to ensure the closing/opening of the socket contacts with no voltage present, a command is positioned inside the interlocked sockets: disconnect switch.


Functional and construction characteristics of the disconnect switch

The disconnect switch is a mechanical device that ensures, in the opening position, a disconnection distance (distance between contacts) such as to ensure safety. This device can open and close a circuit when the interrupted or established current is of a negligible intensity or when the manoeuvre does not produce a significant change in the voltage to the terminals (practically no load). In the closed position, it can carry the normal circuit current and, for a specified time, even an anomalous circuit current, such as, for example, a short-circuit current. Upon closing, it must be able to withstand short-circuit currents for a conventional time of 1s (I cw).


Rated values

The rated values for disconnect switches listed here below are defined in Standard IEC 60947-3.

- rated operating voltage, U e   (V);

- rated insulation voltage , U i   (V);

- rated operating current, I u (A) is the current stated by the manufacturer taking into account the rated voltage, the rated frequency, the rated service, the utilization category and the type of protection container;

- rated conventional thermal current,  I th   (A) ;

- rated thermal current in the casing,  I the   (A) ;

- rated frequency,  f  (Hz);

- normal operation (8 hours or uninterrupted);

- rated closing capacity or making capacity, rated breaking capacity - they are expressed as multiples of the rated operating current depending on the conditions of the service (category of use) ;

- allowable rated short-time current  I cw – is the current that a circuit can carry, without being damaged, in the closed position for a time specified in the prescribed utilization and behavioural conditions;

- rated short-circuit making capacity, I cm   (kA) – this refers to switchgear and disconnect switch for a closing operation on specified peak short-circuit current values. A breaking capacity during a short-circuit is not defined as it is not requested for these devices. When this value is not indicated by the manufacturer, it must be understood as being at least equal to the corresponding peak current I cw ;

- rated fused short-circuit current I cd – the presumed current that a device associated with a fuse can withstand, without damage, for the duration of the latter's operation, under the specified test conditions;

- utilization categories AC – they define the conditions of use and are represented by two letters indicating the type of circuit on which the device can be installed and by a two-digit number indicating the type of use and the intended operating methods;

- mechanical duration and electrical duration – the mechanical duration expresses the number of no load cycles (a single cycle consists of the set of a closing operation and a closing and opening operation) that the device can carry out with no overhauls or replacements of mechanical parts (ordinary maintenance is allowed).
The electrical duration is also expressed in cycles, and represents the ability of the contacts to withstand electric wear under load at the conditions specified by the Standards.

The socket-disconnect switch combination have to guarantee a minimum short-circuit capacity of 10 kA.

Interlocked sockets are equipped with a disconnect switch that closes and interrupts the circuit


4) Possibility to assemble protection devices

Inside the casing of the interlocked socket, one can also install protection devices (fuses or automatic switches), thus obtaining a compact solution for both the functional command (disconnect switch) and the magneto-thermal or differential protection.

Moreover, to guarantee the operators' safety even in case of maintenance operations, interlocked sockets can be equipped with a padlock that prevents the switch closing when the plug is plugged in or prevents the plug from being unplugged when the switch is closed.

Sockets with ground fault circuit interruptor
Switchgear with padlock


5) How can you make sure that everything is operating properly in an interlocked socket?

The most technologically advanced sockets are fitted with electronic devices that signal, by means of lamps, the occurrence of a failure inside the socket or a missing phase that might cause overheating in the motors supplied by the socket.

One example of such a device might be the solution called I-Device:

Electronic device
fitted inside the interlocked sockets


The I-Device is an electronic device (Intelligence Device) that checks the state of the interlocked socket, monitoring the electrical functionality by turning on a pilot lamp located on the socket cover:
- Pilot lamp turned on = Proper operation
- Pilot lamp flashing = Internal socket fault


6) Which moulding technology should be used to make the sockets ?

Interlocked sockets made in heat-hardening plastic material (epoxy resin with fibreglass) guarantee the highest mechanical resistance and resistance to chemical agents.

There are two production processes that cause the product to have completely different characteristics:

SMC (Sheet Moulding Compound) and
BMC (Bulk Moulding Compound) 


this technology uses exclusively non-woven sheets, pre-impregnated with polyester resin and fibreglass. The method consists in preparing the sheet material inside a mould which, equipped with a negative mould, presses the composite so as to allow compaction.

SMC is and advanced technology that enhances the quality of the raw material, and it is highly performing in terms of the mechanical performance of the resulting product (glass fibre length, homogeneity of the material, integrity of the fibres)

On the contrary, BMC technology uses a raw material available in blocks (short fibres and mineral charges), which are subjected to high thermo-mechanical stress during the manufacturing process, consequently diminishing the mechanical properties of the finished products.

Manufacturing process of the component with SMS technology


Results obtained with the SMC moulding technology:

Ultra high resistance to mechanical collisions
The fibreglass-reinforced polyester ensures excellent resistance against impacts even under conditions of low environmental temperatures IK 10 (20 Joule at temperatures of: - 40° + 60 °C )

Resistance to chemical agents
Excellent resistance to aggressive chemical agents, saline solutions, diluted acids, mineral oils and alcoholic substances.

Resistance to atmospheric agents
The double degree of protection of interlocked sockets IP66/IP67 guarantees excellent seal against dust and water, and the reinforced polyester material ensures high resistance to UV rays.


7) Which type tests must be carried out on interlocked sockets?

The type tests required by the reference standard listed here below must be carried out on interlocked sockets:

IEC 60309-4 : Plugs, socket outlets and couplers for industrial purposes.

The most important tests pertain to the electrical duration, the Short-Circuit/Normal operation test, heating test with rated current and IP test (Degree of protection)


• Short-circuit and Normal Operation test

The electrical characteristics required by the standard for the short-circuit test are listed here below:

The electrical characteristics required by the standard for the normal operation test are listed here below:

The short-circuit and normal operation tests are carried out in order to ensure that he product is able to withstand the thermal and mechanical stress of operation under extreme conditions (short-circuit) and to ensure operation during the repeated opening-closing cycles (electric duration), even with high voltage/current phase displacements.

Testing machine for the short-circuit and normal operation tests


• Heating test

The heating test is carried out in order to verify the maximum over-temperatures reached by the various socket components during operation with rated current.

The critical components are the connection terminals of the cables and the contact pads.

The parameters to be used for the heating tests are shown in the table here below:

The heating test is carried out on the "aged" product, in other words to the product subjected to the short-circuit and normal operation test, so as to verify the state of contacts and terminals after a typical lifecycle.

The maximum overt-temperatures allowed on the terminals must not exceed 50°C.

Testing equipment for rated current heating tests

Graph showing the over-temperatures obtained by the heating test on interlocked socket) Heating test .


• Testing the degree of protection of the case (IP Degree)

In order to ensure the operator's safety and that of the electrical connection, the casing of the interlocked socket must guarantee suitable IP protection (against dust and water), for heavy-duty industrial uses, the protection to be guaranteed is IP66/IP67.

Degree IP66 means subjecting the product to a high-pressure jet of water and to a dust seal test with casing in depressure, while the IP67 test entails immersing the product in water (height of 1 m ) and verifying that no drops of water have penetrated inside the casing.

Degree of protection test: IP66/IP67


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