General Overview
This a key which was the standard for single lever keys in the BPO.  Known by Ericsson as the N5300 Series.

Used in a variety of light-current switching applications and supplied with various spring combinations within the stated limits.

Consist of a metal frame carrying two banks of insulated contact springs which are operated by two rollers attached to a pivoted lever.  Double-throw keys embody a device preventing roller rebound.  The key can be fixed via 4 screws to a mounting plate.

The cam of a standard lever key has a 2BA thread for a length of 5/8" and this accepts the handle.

The Key is fixed to the mounting with four screws, No. 7BA x ¼", Countersunk head, MS, matt black.

Uses Handles No. 1.  They come in various colours and screw onto the lever.  There are two types, diamond finish and fluted.  As from 1960 onwards only fluted was supplied.

Switching Action
Keys are single or double-throw with locking (L), non-locking (N) and stop (S) actions.

Contacts are normally silver.

Contact Units can comprise of:-
Make (M), break (B), changeover (C), or make-before-break (K) combinations.

Small frame (Figure 1)
Two sets of three springs on the frame side (1st side). Two sets of 9 springs remote-from-frame side (2nd side).

Large frame (Figure 2)
Two sets of six springs on the 1st side. Two sets of 9 springs on the 2nd side.

Contact Rating (Silver)
300 mA at 250Va.c. (r.m.s.)
100 mA at 250Vd.c.
300 mA at 100V d.c.
Note: Contact ratings given are for non-inductive loadings only. Adequate spark quenching must be applied when connecting into inductive circuits.

Key No. 298
Showing springs

Issue 1 (2/3/1960)


1. Introduction. This Instruction is intended to assist designers in the selection and use of standard lever keys. The construction, performance and range of keys available are given together with the methods and space required for mounting them. Associated components, pin retaining plate and dummy keys are also described.

2. Typical keys are illustrated in Figs. 1 and 2. Fig. 1 shows a single-action (i.e. single-throw, two-position) non-locking key and Fig. 2 a double-action (i.e. double-throw, three-position) key, non-locking on the frame side and locking on the side remote from the frame. Non-locking indicates that when released the key restores to normal by the pressure of the contact springs on the rollers, and locking that it remains operated until restored manually.

3. In keys with mixed actions, the non-locking springs are normally placed on the frame side of the key (and are operated when the handle is moved away from the frame) and the locking springs are placed on the remote side.

4. Double-action keys were originally designed to be 'clickless' i.e. they side and cause momentary operation of not possible to guarantee this feature the facility without first consulting should not overshoot on restoration from one the contacts on the opposite side, but it is on all keys and designers should not rely on the facility without first consulting S Bch. (S212).

FIG. 1 FIG. 2

5. The four types of contact-unit are given in Table 1.

6. In describing a key-action in abbreviated form the action on the frame side is given first. In addition to the abbreviations given in Table 1 the following letters are also used:-
S - (stop) indicates no action on that side
L - indicates a locking action on that side
N - Indicates a non-locking action on that side

The key in Fig. 1 is N2C-S and that in Fig. 2 is N2K1M-L2B



7. Tables 2-8 give the range of standard lever keys available, classified according to their action (single or double, locking or non-locking) with a diagram of the contact arrangement (with frame on the left-hand side), spring numbering (see Diagram ATW 22001), drawing number and key-group (see par. 21). Designers should use keys from this range even when the smallest suitable key has more contacts than are necessary.

8. Keys are now issued by the Supplies Dept. without handles, which should be requisitioned separately. Handle, Key, No. 1 ... (Colour) is the standard handle. In new drawings, and as opportunity affords when existing drawings are amended, keys and handles should be called up separately.


9. Rating. Maximum ratings for non-inductive loads are as follows:-
300 mA at 100V a. c. (r.m.s.)
300 mA at 100V d.c.
100 mA at 150V d.c.

10.  Insulation resistance. The insulation resistance between any two points not intended to be in electrical contact is not less than 1000 megohms when measured at 500V d.c.


11. Weight. The larger keys weigh about 5½ oz. and the smaller about 3½ oz.  Where weight is an important factor, designers are recommended to consider other switching devices such as rotary wafer switches.

12. Life. A key should still operate satisfactorily after 300,000 operations with the contacts carrying 300 mA at 100V d.c.

Single Action Keys, Locking

Single Action keys, Non-Locking


Double Action Keys, Non-locking both sides


Double Action keys, Locking both sides

Double Action Keys, Frame side No-locking, Remote side Locking



13. Figs 3 and 4 are "exploded" views of the keys shown in Figs. 1 and 2 respectively, and their component parts are described in the following paragraphs.

FIG. 3

FIG. 4

14. Frame. The frame is made of brass or die-cast zinc alloy in three patterns which are used as shown in Table 7. Four holes are drilled and tapped in the top of the frame for the fixing screws and a further four holes drilled for studs (Parts 3/SST/306), two of which are fitted on each non-locking side.


Frame Key Use FIG.
Type No.
Small 1/SF3/36 Single Action With springs on remote side or not more than 6 springs (three in each pile) on the frame side 5
2/SFR/36 Single Action
or Double Action
With not more than 6 springs (three in each pile) on the frame side 6
Large 1/SFR/39 With not more than 12 springs (six in each pile) on the frame side 7

FIGS. 5, 6 and 7

15. Cam. The cam is made of mild steel in two patterns: Part 1/SCA/54 (Fig. 8) for single-action keys and Part 2/SCA/54 (Fig. 9) for single or double-action keys. The top of the cam is threaded so that the handle can be screwed on. The mild steel axle (Part 1/SAX/5) is fixed rigidly to the cam and its ends are spun over to retain the ebonite or moulded rollers (Parts 1/SRO/1) and washers (Parts 17/SWA/34). The washers may be omitted provided that the rollers are free to rotate but it is not essential that they do so when the key is operated. On single-action keys a mild steel stop pin is rigidly fixed to the cam on the 'stop' side. The two cam assemblies for single-action keys (Part 1/SCA/55) and double-action keys (Part 2/SCA/55) are shown in Figs. 10 and 11 respectively. The steel bearing pin (Part 1/SPI/1 or Part 4/SPI/7) is fixed rigidly to the cam and rotates in the bearing holes in the frame.

FIGS. 8, 9 10 and 11

16. Double-action keys are provided with back spring (Part 1/SSP/638), stud (Part 1/SST/200 for small frames, Part 2/SST/200 for large frames) and steel ball. The ball engages In the notch in the cam (this part of the cam edge is hardened) to bring it to rest when the cam Is-restored to its central position and thus assists in providing the 'clickless' feature (see par. 4).

17. Spring-piles. The spring-piles are built up with nickel-silver springs (brass is an alternative for thick springs) with riveted or welded silver contacts and ebonite or SRBP separators (Parts 29-33/SSE/33). The separators are 1/32in., 3/64in., 1/16in., 1/8in. or 1/4in. thick and thus permit any spacing from in. upwards in steps of 1/64in. Each pile is fixed to the frame by two screws (Parts 1 -/SSC/179) contained in ebonite bushes (Parts 1 -/SPU/90) with a common plate (Part 1/SPL/91) at the outer end of the piles to keep them in alignment. The screws and bushes are made in a range of lengths so that the screws fully engage the threads in the frame but do not project more than 1/16in. beyond it.

18. Pin retaining plate. The plate (Part 1/DPL/640) is shown in Fig. 12. On old keys, the cam bearing pin sometimes becomes loose in the cam. To avoid changing the key when this happens (if the key is otherwise satisfactory) the pin should be pushed back and one of these plates fitted between the top of the frame and the mounting plate or panel. The side-pieces of the plate will then prevent the pin coming out of the frame.

It should be noted that the use of this plate increases the effective width of the key from 0.743in, to 0.781in. and the minimum side-by-side spacing of keys is correspondingly increased. The plate cannot therefore be used on key-mountings using the minimum separation of ¾in. between drilling centres, but in such cases the pin should be kept in place by the keys on either side and if required at either end of the mounting, one of the side-pieces of the plate can be removed.


19. For standard key-mountings see H 1020 which gives the key-groups (see par. 21) for which each mounting is suitable.

20. When mounting keys on panels 0.080 in, thick use drilling 1/DR 502 (Fig. 13) and on panels 0.125 in thick use drilling 1/DR 516 (Fig. 14). Keys are fixed to the panel by the four 7BA countersunk screws supplied with the keys and can be mounted in any position.

FIG. 13 - Drilling 1/DR 502


FIG. 14 - Drilling 1/DR 516

21. For mounting purposes, keys have been arranged according to size in a number of groups which are shown in Tables 2 to 6.

22. Any key can be mounted in a space ¾in. by 2 in. on a panel and extending 3 in. behind it. Further details, required particularly when key bars are used for mounting, are given in H 1020. For most keys however, the 2 in, dimension can be reduced, and Fig, 15 in conjunction with Table 8 shows the mounting space required for keys in each group. The tags are spread after assembly to facilitate wiring and on the larger keys this necessitates a clearance for the tags on the remote side greater than dimension A (Fig. 15). The tags can quite easily be bent back a little, if necessary, and without introducing any undue difficulty in wiring.


Key Group Dimension A (in.)   Key Group Dimension A (in.)
S1 0.47   L1 0.47
S2 0.51   L2 0.65
S3 0.63   L3 0.91
S4 0.76      
S5 0.82      
S6 0.88      
S7 0.95      


FIG. 15




23. These are plates designed to cover any unused drillings on multiple key mountings and have a raised bar to fill the slot in the drilling. The plates are fixed by four screws (supplied with the dummies) in the same way as keys. The two types of plates are detailed in (a) and (b):-
(a) Key-dummy No. 1 (Fig. 16) is of brass, lacquered black or of black moulding material and is for general use.
(b) Key-dummy No. 3 (Fig. 17) is of SRBP and is for use with Keys No. 212.... when less than four keys are fitted.


24. The specification for standard lever keys is D 484 and the drawing number for each key is given in Tables 2 to 6.

25. Other Instructions which may be of interest are:-
H1010 - Key handles and stops for standard lever keys.
H1020 - Standard key mountings for standard lever keys.

26. Queries on points not covered in this Instruction or in the above documents should be addressed to Eng. Dept. (S2/2).


BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: February 18, 2021