RELAY-SWITCH No. 3106 N.L. 1-15

Introduced soon after 1938 these relay-switches are made of iron with two interlocking covers.  The purpose of this relay-switch is to isolate low voltage telephony circuits from mains voltages.

The units consist of a 3000 type relay, rectifiers and up to two 15amp mercury switches (Tubes, Mercury. No. 1) of the make variety.  Where only one mercury tube is fitted, a Tube, Mercury, No. 1, Dummy is fitted in the spare position.  Each relay has a standard C/O contact (pt) fitted.

The rectifiers are wired when required.  They are generally used when the relay is operated from a low voltage AC source.

Now days this can all be done with discrete components - click here for more information.

Safety Warning
The glass tubes have the metal Mercury in them.  This is a dangerous liquid chemical and medical attention should be sought if any contact is made with the human body.

Title Description
There are numerous variations of the Relay-switches and each is described by the title number.

The title number is interpreted as follows:-

Name - Type number - locking (L) or non-locking (NL) - quantity and rating of mercury tubes
                                                                                                  relay cover type

Referring to the title at the top of the page - this describes a relay-switch with a relay of the 3100 type, which is non-locking and includes one tube, mercury of 15amp and the unit fitted with a AW cover.

The No. 3106 is fitted with a Relay, No 4309.

AW - Used in subscribers premises.
AX - Used in exchanges.

The unit is separated into two compartments by a partition across the middle.  This partition has a sliding rod through it, which is operated by one side of the relay armature.   The relay is located in the lower compartment.  The other side of the relay armature operates a standard contact set.  The rod, when the relay is energised, moves upwards pushing a spring loaded lever, in the upper compartment, upwards.  The end of the lever is located in a swinging arm which holds the mercury tubes.  The swinging arm rotates and the mercury in the tubes cover the contacts, making an electrical circuit.

When the relay is de-energised the spring in the upper compartment assists in restoring the swinging arm back to it's home position.





The upper section is where the mains cables enter the unit and are terminated.










The lower section section are where the telephony cables enter the unit and are terminated.  The low voltage relay and rectifiers are also in this section.


The picture above is dated 1938.  This is a Siemens made switch unit with two mercury tubes.

It had two covers, one to cover the mains section and one to cover the telephony section.

The steel partition in the middle segregates the two sections.




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Last revised: November 20, 2021