Exchange 'changeovers'
 

There were usually two reasons for a 'temporary' or relief exchange.. The first was if the existing exchange had been damaged for some reason - such as fire. The other was if the existing exchange became too small. In the former, the lines would be moved as fast as possible from the underground cables feeding the existing exchange being disconnected and reconnected to the cable to the new temporary (usually a 'mobile' exchange in a large trailer that could be moved quickly). For a relief exchange to an exhausted one, it was often a mobile exchange if only for a couple of years but on other occasions, a wooden building would be put up beside the existing exchange to house another exchange. When the original exchange was extended or replaced, each line would end up being connected to both exchanges but with 'plugs' or wedges disconnecting the lines from the new exchange. Then on the day of the change-over at the 'given time' the lines on the old exchange would have wedges pushed in to disconnect them then the wedges/plugs would be pulled out to connect the new exchange. Then there was the long job of carefully disconnecting the unneeded wiring known as 'jumpers' on the Main Distribution Frame.

This is a picture of the BT engineers changing the lines over one at a time on the last electro-mechanical exchange in the UK public telephone network. The 'Main Distribution Frame' (usually a large rack of 'tagblocks' as in second photo) was just a small partially equipped 100 pair 'Krone' connection box on the wall! The exchange (an Island Automatic eXchange No 5 - IAX5) only had 20 lines on it and they were still dialling two digits to reach each other and had untimed local calls in July 1995 ! The exchange is the rack on the right of the picture. The exchange was on the remote island of Foula (population about 30) way out in the Atlantic to the west of the Shetland Islands - so remote that all calls to/from the island were trunk calls! Exchange recovered and now back here in North Wales.

The other picture shows the change-over of Crawford SAX (Small Automatic eXchange) several weeks earlier which BT thought was the 'last electro-mechanical exchange' !! I was the only one there that knew it wasn't! This change-over had the new digital exchange lines 'wedged out' on the Main Frame. At the appointed time (10.30 in the morning) the cables to the old electro-mechanical exchange were cut by one on the BT engineers (by the name of Bob Hope !!) and then two children from the local primary school pulled the wedges out to connect the lines to the new digital 'exchange'.

It took quite a while on a 10,000+ line exchange!

Ian
CNet 0352 2979
MR ETD 053-6278

Re: Exchange 'changeovers'
 

I remember seeing 'cutover' done at stupid-hundred-hours-on-a-Sunday-morning on an exchange in east London back in the 1970s. All the wedges were tied together with cords, these cords were then tied together in bundles, from each bundle a thinnish rope extended, these thinnish ropes were then attached to a thick rope.

At the appointed moment a couple of guys heaved on the thick rope!

Re: Exchange 'changeovers'
 

I don't know how they accomplished the switchover of the Etwall exchange from 4 to 6 digits at the tail end of the 1980s; but as an incurable experimentalist, I could not resist lifting the receiver during the advised fifteen minute window of unavailability. The old "purring" dial tone was replaced by silence for several minutes, then the higher-pitched modern dial tone. And no need to dial 9 to call Burton-on-Trent anymore! 93 (formerly 993) still got you to Derby, although the dialling code page in the phone book showed the "proper" code 0332 (as it was then) for Derby.

Re: Exchange 'changeovers'
 

Is the kit installed in these tiny, remote exchanges in the 1990s or earlier still in service?

Re: Exchange 'changeovers'
 

BT made a great effort once 'Strowger' and the earlier non-digital TXE2, TXE4, UXE7 and UXE8 exchanges were out of service to recover them by the year 2000. Luckily some of us had the forethought to 'acquire some Strowger kit - I recovered several exchanges from Scotland and Wales and racks from several exchanges from Yorkshire to Devon between 1992 and 1997. As BT no longer own the buildings (they only lease back what they need) they tried to clear out what wasn't needed. The smaller buildings no longer house an 'exchange' but a 'remote concentrator' which concentrates a number of lines down to effectively a few circuits needed to complete the number of calls required at any one time. For instance 'Canterbury' and 'Dover' just have concentrators with the equipment which processes the calls at a location further west. As the modern equipment is a lot smaller, BT will end up with concentrators in roadside cabinets. I remember when Crawford was being changed over, the TV interviewer asked the BT Press Officer ' where have you built the new exchange?' (The old electro-mechanical consisted of three suites of 8ft 6in high racks packed with clicking/whirring equipment.) The answer was 'Its there in that cabinet behind the entrance door' - a double door unit the size of an office stationary cupboard !! There might be the odd rack that survived but unlikely.

Ian
CNet 0352 2979
MR ETD 053-6278

Re: Exchange 'changeovers'
 

At the time of the change-over Etwall was a 'Non-Director' exchange becoming a System Y/AXE10 exchange and it would have taken a while to 'peg out' the thousand odd lines/ cut the cables on the old exchange before pulling out the wedges to bring the new one into service.

Prior to 1969, Etwall was a UAX13 which had the route to its parent exchange - Burton on Trent - reached by dialling 9.

However it also had a direct route to Derby - presumably reached by dialling 8 - the direct code from Derby was 963 whilst Burton on Trent was 962. I wonder if that code disappeared when the Non-Director exchange came into service in 1969?

Once it went digital and became part of the Burton-on-Trent 'Linked Numbering Scheme' there would be no need to dial the '9' as the initial digits of the 5 or 6 digit number indicated the exchange required.

Some of the old 'Strowger' UAX13's were in Linked Numbering Schemes - I managed to acquire the last racks that provided that facility from Grosmont UAX13 near Whitby which had been in the Whitby LNS with 85xxx numbers rather than the usual three digit numbers of a UAX13. There were only a dozen or so such exchanges in the UK, plus a few in Northern Ireland. Only introduced in the late 1970's! The racks I have are dated 1980!

Into the 1980's it became the practice to publish STD codes even for 'local charge' calls to exchanges as everything was going digital and in preparation for when charging became 'per second'. However the '93' route would still be required for use by the other exchanges parented on Burton-on-Trent until they were changed over.

Ian
CNet 0352 2979
MR ETD 053-6278

Re: Exchange 'changeovers'
 

Thanks for the explanations of how the cutover was done - another mystery solved!

Re: Exchange 'changeovers'
 

Hi, there's an interesting video of an exchange cutover here

Andrew

Re: Exchange 'changeovers'
 

I don't remember 8 for Derby; it was always 993. Etwall (73 nxxx), Hoar Cross (75 nxx), Barton-under-Needwood (71 nxxx) and Sudbury (78 nxx) all used the same codes as each other, 9 for Burton-on-Trent and the same codes as Burton but with an additional 9 in front.

There was also a Marchington sub-exchange (72 nxx? Memory faulty) whose numbers were absorbed into Burton-on-Trent sometime in the late 1970s / early 1980s.

Derby to Burton was 962; Derby to Hoar Cross was 96275, Derby to BuN was 96271, Derby to Sudbury was 96278 but Derby to Etwall (the only one in Derbyshire) was 963 (although 96273 also worked). It's possible that if Etwall was originally a 3-figure exchange that was upgraded to 4 figures before my time, the old 3-figure Etwall exchange might have supported 8 as a shortcut for Derby but this was not retained for some reason. The inbound shortcut from Derby would not have been affected.

Re: Exchange 'changeovers'
 

I've seen a preserved Strowger exchange at the Dean Forest Railway in Lydney, Gloucestershire and am pleased to hear others have also been saved.

I'd heard of remote concentrators but thought they were confined to villages and suburbs. It's interesting that calls in medium sized towns are now processed elsewhere.

Re: Exchange 'changeovers'
 

At least five preserved railways are still using Strowger - Talyllyn, Kent & East Sussex, Bluebell, Peak Rail, Swanage with several inter-connected via the Minor Railways ETD (Extension Trunk Dialling) phone network with dialling codes based on the British Railways ETD network of the 1970/80's.

And many of the preserved Strowger (both ex-GPO public and PABXs plus former private PAXs) are connected to CNet - our replica of the UK public network of the 1960-80's.

Concentrators have been around since the 1930's - initially in the form of the Country Satellite eXchange (CSX). They only worked off auto-manual switchboards/exchanges. Lasted into the 1970's in Scotland. Then came the Line Connector No 1 (two links plus ten lines) designed for pole mounting (I managed to acquire one in the 1990's from a pole in a scrap yard!) and the Line Connector No 2 (four links and twenty two lines) mounted in roadside cabinets. The last No 2 I remember in service was in the early 1980's but the No 1 didn't last long as one call could 'block' it.

Ian
CNet 0352 2979
MR ETD 053-6278

Re: Exchange 'changeovers'
 

I was involved in a couple of these changeovers. Box(Txe2 to AXE10) and Chipping Sodbury( Crossbar to AXE10).

It was more or less as stated, with the "new "exchange in and working, and jumpers teed onto customers bar pairs. At the appropriate moment(07.00 ISTR) a group of engineers up on the overhead cable grid cut off(literally) the old exchange by cutting the cables down to the bar pair blocks, while another group pulled bunches of strings attached to plastic wedges that then connected in the new exchange.
At Chipping Sodbury I missed the fun of pulling the strings as I was in the former group as explained above, but I did do it at Box.

In those days it was traditional for the manager to cook us all a fried breakfast upon completion of a successful transfer.

Re: Exchange 'changeovers'
 

The changeover was prepared for some weeks( sometimes months) in advance, depending on the size of the exchange and number of lines it served.
We had a device that connected to the test systems of both exchanges, and this would run through each line and check the completed jumpering was correct, ie, a number on the "new" exchange was connected to the right place, ie. the same number on the "old" exchange.
Small exchanges could be jumpered one at a time, but for larger sites we made up hanks of jumpers( a bit like a wiring loom-according to a prepared chart) with each pair labelled at each end so that when the form was run in, the jumpers could be connected to the correct place.

Re: Exchange 'changeovers'
 

I wonder if one of those has any equipment from Westbury -on-Severn.

Early in my exchange construction career a workmate and I were given a job to recover a rack of ( supposedly) redundant equipment from Westbery-on-Severn in Gloucestershire sometime in the early 1990's. We weren't sure the rack was actually dead, and that our information was correct, but after a phone call to the planner and our boss we were assured it was.
An hour later we waved the rack off as it was driven away on a trailer to it's new home at a preserved railway, and retired to the local hostelry for "refreshments" (you could in those days). As we drove back past the exchange there must have been about a dozen vans outside. Naturally curious we asked what was happening, only to be informed that we had inadvertently cut off the whole exchange, since DC power to the system X( I think) was connected through our rack.
A few comments such as "I wonder who did that then", and such like were passed, as we tried to keep a straight face. Luckily for us the information in our job pack was incorrect so we were covered, and I can laugh about it now.

And then there was the one about the meter rack at Stratton-on-The-Fosse , but that's for another time.

Re: Exchange 'changeovers'
 

I remember some line concentrators serving some rural locations running up until the 1990's at least .
I think, as Ian said above the customer end was pole mounted and had about 5(or 10) "trunks"( or exchange lines) which would be switched in by a unit in the exchange. I think there must have been a control line too, for the "data", such as it was, to signal the "conc" to connect incoming calls to the correct customer. If one, or more , of the trunks went faulty then it was quite tricky because often customers wanting to make a call, or callers got busy tone, as sufficient lines were not then available if a few people were on the phone at once.
I wasn't involved with the kit at the pole end, but did jumper up a few circuits onto the exchange end of the line concentrators.


I ought to write all this down you know. I think I'll call it "From Strowger to Superfast", confessions of a telephone engineer.

Re: Exchange 'changeovers'
 

Those following this thread might find the website of the Dean Forest Railway's telecoms team of interest http://www.dfrtelecoms.org.uk/tmainmen.htm

Re: Exchange 'changeovers'
 

Last time I was at Churnet Valley railway they had a Strowger mobile exchange (the green caravan) installed at Froghall station. The dialling sequence for an outside line was written on each phone, quite a sequence from memory.

Re: Exchange 'changeovers'
 

The preserved railways still with Strowger -
Talyllyn - 50 line GEC SE50 PAX at Tywyn (Pendre Works) and 22 line GEC uniselector PAX at Abergynolwyn that I recovered from a salt mine about 25 years ago.
Kent & East Sussex, MNDX at Northiam and PAXs at other stations
Bluebell - not visited :(
Peak Forest - Strowger exchange based on an ex-GPO 'expediency' rack on an MNDX trailer at Rowsley.
Churnet Valley - MNDX (Mobile Non-Director eXchange) at Froghall and UAX13 at Cheddleton
Gloucester & Warwickshire - two 50 line AT&E PAXs.
Dean Forest - UAX13

Some are interconnected by the 'Minor Railways ETD' (Extension Trunk Dialling) system based on the codes used by the British Railways 'Strowger' ETD system of the 1970's/80's

Ian J
CNet 0352 2345
MR ETD 053-6278

Re: Exchange 'changeovers'
 

I was involved in a number of "changeovers" when system X and AXE10 were being introduced. the main thing was it was done early on in the morning after 0800 so that the customers could test it out and report any faults while the engineers were still on site, people who had reminder calls setup or call diversion were a problem. and the big sit down breakfast provided for the engineers who switched it over.

Bigger changovers were when the trunk switches were introduced, but these were very early in the morning so that the routes could be tested. the change over was made by deactivating the 2Mbit C7 links on the old system and activating the stm4's on the new system.

I remember a software problem at bishops stortford that needed the reversion of the change over to get the trunk back up

Re: Exchange 'changeovers'
 

Thank you Andrew,
I watched the film last night. I wondered how they switched over the systems with only a very short down time.

Bob.