Could some kind person(s) confirm that the fuses in both the CF3 and the CFR cut-outs should be 25A? I have an example of both of these and they both have what look like original Lucas brass-ended fuses in them - the CF3 fuse and its spare are both 25A, also the 2 fuses + 1 spare in the CFR are likewise 25A.

Also, how many types of cut-outs were fitted and when? My 1925 car has a small cut-out with no fuse and CAV embossed on the lid. I think that the next type (from 1926? to ??) was the CF1? Then the CF3 and finally the CFR?

TIA, David

If I might expand the question a little, does anyone know the fusing characteristics of the original lead strip fuse in the CF1, and am I anywhere near with a piece of "15 amp" open wire?

Martin

Hi David,

I have just had a look at my car (October 1925) and it has the small cut out with no fuse and C.A.V embossed in the cover. The cover is made of steel, but I believe the first ones were of aluminium.

Lance.

I can advise you Martin that my 1931 car was fitted with an flat strip of alloy which will blow at EIGHT AMPS. This could be replaced with TEN AMP fusewire but the dynamo should be adjusted on the third brush not to exceed an output of eight amps as it may burn itself out. This work was completed in 1979 and I have quoted from the invoice for that work. I have not experienced any problems other than recent cut out switch regulation which I am about to change for an electronic unit.

Can't at this stage assist with dates and ratings but the sequence was :
Early CAV type E ( with aluminium cover as Lance says ) later CAV type E with steel cover,Lucas CF1, Lucas CF3,Lucas CFR and finally Lucas CFR2.

May I confirm Davids comments. I can confirm that that the flat fuse was 8 amps . And it was practice to leave a couple of spares connected at one end. When these go there were no replacements. I have the same set up. I have to use 10 amps and have done for years. I have set my dynamo to give 8 amps at what I consider to be my cruising speed. I have seen cut outs with a domestic 35amp fuse!!

Thank you all for the replies - however, I would still like to know for certain what size(s) of fuse were used in the CF3, CFR and CFR2 cutouts. I would be grateful if Box/Ruby, etc owners could have a quick look next time they're in the garage (please?). The trouble is that the Handbooks just give rather unhelpful advice like "Never fit any fuse other than the standard Lucas fuse as originally fitted. The size of the fuse is marked on a coloured paper slip inside the fuse." The CFR & CF3 cutouts I have do have fuses like this - they are all 25A, but I thought this value was rather high.

David

David

I seem to recall reading some time ago - can't remember where now - that unlike domestic and almost all other applications where fuses are marked with the normal current carrying capacity, automotive fuses are marked with the current that will melt the element and have a fusing factor of 2. If this is true then the fuse marked as 25A is designed to carry a current of 12½ amps when installed in the intended mounting. This would appear to be a reasonable value to me given the normal output of A7 dynamos.

In my post of 2nd Oct I was seeking the fusing characteristics of the flat lead strip type, ie time to melt at various current values rather than the figure marked (or not marked) on it. Short of testing the fuse in question I somehow doubt we will ever have a definitive answer to this since very few, if any, of the original fuses survive. If anybody does have an original lead fuse (or the remains of one) it would be interesting to put a micrometer on it and try to replicate in order to test.

Martin

Hi David,

Unfortunately a cut-out unit wasn't included in the box of bits that came with my 'basket case' A7 rolling chassis. However I've owned a 1936 Series I Morris Eight Tourer for 35 years and the Eight's were similarly fitted with CFR2 6v units - dynamo was third brush regulation. The Morris Motors Handbook (1935 edition) states that the Auxiliary Field Fuse is 25 amps and the Dynamo Field Fuse is 5 amps. Many years ago I bought a N.O.S CFR2 unit in its original Lucas box and the brass ended fuses have the Lucas pink paper slips inside the fuse. This unit is a later CFR2, L1, 6v, part number 332151, dated 12 53. The Auxiliary Fuse is again 25 amp but the Dynamo Fuse is 8 amp.
Hope this may be of help.

Jeff

Here is a link to some strip fuses

David, the only place I have seen reference to fuse ratings is in the sixth edition (1937) of The Austin Seven Book by R T Nicholson. It may be in earlier editions which I don't have.
Dynamo field............6 amps
Accessories.............25 amps
Dipping reflector.......10 amps
Trafficators............25 amps.
This relates to the Ruby fitted with the CFR2 cut out/ fuse box.
John Comforth has drawn a really user friendly Ruby wiring diagram which I found on a Austin Seven website (possibly Dorset) and this shows similar fuse ratings.
I think it is safe to assume that these ratings also apply to the CFR and CF3 cut outs although in the latter case the field fuse lives in it's own housing on the dynamo.

Hi: Can someone in this forum help me find a CFR 2 cut out unit. I'm restoring a Morris 8 Series 2, 1938. Tom Simpson

Location: san francisco

Tom, my first port of call would be Tony Betts at 7 County Austins 0116 2867522.

Ian Mc.

Location: Shropshire

As far as the dynamo fuse is concerned, care is necessary applying the fuse value of one wiring system to another. For post 1933 cars the fuse seems to protect primarily the summer resistor, and carries only the low summer field current.

On some earlier circuits the fuse carries dynamo output. Possibly intended to protect primarily against a stuck cutout and reverse current.

Location: Auckland

I have a copy of a booklet called "Electricalities" published in about 1938 by Lucas. In the section on the CFR2 it states "The Fuse on the right hand side is connected in the dynamo field circuit and will blow in the event of the battery becoming disconnected from the dynamo due to broken wiring or a loose or broken connection at the dynamo or battery terminals". It goes on say of the Auxiliary Fuse "This fuse will blow in the event of a short-circuit occurring in the wiring of any of the accessories connected through it, or in one of the accessories themselves".

In my 6th edition (1937)copy of Nicholson there is a Wiring Diagram No W81021 for Austin Seven Cars (1936-7) which I assume to originate from Lucas. The Fuses are listed as Dynamo Field 6 amperes, Accessories 25 Amperes, Dipping Reflector 10 Amperes and Trafficators 25 Amperes. The same values were used on a 1938 Hillman Minx (Wiring Diagram No. W63160 in the back of "Electricalities"), except for the Dynamo Field which was 8 amps to suit a higher rated C45 dynamo. This is the best information I have, and was therefore what I showed on my circuit diagram on the Dorset Austin Seven Club website.

Location: New Forest

At the risk of leading off on a tangent (the vice and virtue of the Forum), despite having been a radio technician in a former life, I struggle to grasp the purpose of the field fuse in the post 1933 cars. It may blow if the battery became disconnected, but only if Summer charge selected. On Winter charge it is not in circuit, the dynamo functions without it.

It has one use in that if Summer is selected and the fuse removed the dynamo is switched off; enabling relief for the battery on 300 mile day journeys!

Otherwise it seems only to protect the summer resistor, presumably from some freak condition or meddlesome action where full battery may be applied between isolated D and F cutout terminals.

Have I missed something?
Can anyone offer a more rational explanation for the fuse please?

The circuit was adopted for over 100,000 cars so presumably has some sound basis.

It is worth remembering that Sevens are not fused as thoroughly or as sensitively as moderns. A lot of wiring is very live all the time. And the Auxillary fuse allows a lot of current. Worth remembering with the car parked in a garage alongside the Bugattis, or in a basement under your bedroom.

Location: Auckland