Upthread I said:
“Dynamic braking – EMD field loop vs. GE potential wire – somewhat resolved with dual systems in the mid-1950s, probably led by railroad initiatives more than the locomotive builders, then finally addressed by EMD’s adoption of potential wire control in 1961, with retrofits available for older units. Incidentally, this was never a problem in export markets, as EMD adopted potential wire control for export models from the start. There compatibility did not seem to have been the primary driver; rather the field loop control was found to be functionally less suitable for the export models.”
An example of a railroad initiative was that of Western Maryland, which modified its Alco-GE units (fitted with potential wire dynamic brake control) and its EMD units (fitted with field loop dynamic brake control) so that they could work in mixed MU sets with either exercising full control of the dynamic braking of the other.
How it was done was described in an article in “Railway Locomotives and Cars” (RLC), 1957 April, p.68ff, “Western Maryland Ups Locomotive Utilization…Alco and EMD Units Run M-U”.
EMD’s adoption of potential wire DB control was recorded in RLC 1961 April, pp.40,41, “EMD Field Loop Can Be Eliminated”. Details were provided as to how older EMD units could be converted from field loop to potential wire braking.
EMD’s earlier use of potential wire dynamic braking in export locomotives was cover by US patent 2745050 of 1956 May 08, filed 1952 January 02.
Evidently Fairbanks-Morse claimed to be the first locomotive builder to offer “universal” dynamic brake control as a factory fit. This was recorded in the book “Train Master” by Diesel Era with David R. Sweetland, Withers 1997, on p.53. It applied to the 1956 Train Master batch built for the Southern, fitted with GE Amplidyne equipment, and which were required to operate in MU with Southern’s existing EMD F7 fleet.
Something I haven’t been able to determine is when Alco first offered “universal” dynamic brake control as a factory option. (It was available on the GE U25B from the start, as far as I know.)
Thus was the dynamic braking control incompatibility variously resolved on the pathway to a standard diesel MU system.
Cheers,
The previously discussed Milwaukee Wylie throttle used to allow DC electric locomotives to control trailing diesels was an early example (electromechanical era, one might say) of mapping from many notches to eight diesel-electric notches.
From the same era, the EMD FL9 dual-mode locomotive of 1956 provided an example of mapping the other way, allowing control of what was effectively a multinotch DC electric locomotive from an eight notch diesel controller.
The details were provided in the book Diesels to Park Avenue, by Joseph R. Snopek and Robert A LaMay, as follows:
Diesel Notch Electric Notch
1 Series-parallel manual 1
2 Series-parallel manual 2
3 Series-parallel manual 3
4 Series parallel manual 4
5 Series-parallel automatic steps 1 to 17. Last step (17) is full series-parallel.
6 Parallel automatic steps 18 to 26. Last step (26) is full parallel.
7 Parallel automatic step 27, first field shunt.
8 Parallel automatic step 28, second field shunt.
The automatic steps were under the control of a current limit relay, whose setting was variable via an engineer’s control. Automatic acceleration was needed to 'condense' many notches into few. It was a well-established technique in EMU practice, where often just four controller notches were used. So one could say that EMD expanded it to eight notches.
Thus counting both the manual and automatic steps, the DC control involved a total of 32 notches, of which four, namely 5 through 8, were running notches. My guess is that the second field shunt corresponded to field shunting in the diesel-electric mode, with the first step included to minimize the magnitude of the notching current spike. In the diesel-electric mode, the running back of the load regulator took care of that.
The FL9 case may be compared with that of the New Haven EP-5 class electric locomotive when operating in DC mode. Therein it used 31 of the 35 controller notches, all manually switched, with just two being full-field running notches, field shunting not being used on DC.
Mentioned in my June 26 post as one of the sidebar aspects of the AAR MU system was:
“The UP system that allowed leading GTELs to control trailing diesel-electrics. There seems to be no detailed information available as to how this was done.”
Starting in 1958, most but not all of the UP GTEL4500s and GTEL8500s were fitted to MU with trailing diesels. It appears to have been a UP initiative; whether GE was also involved is not known.
I have not been able to find any information as to how the 20 notch GTEL throttle control was mapped to the eight diesel notches. Maybe that level of detail has never made its way into the railfan domain. My guess, and it is only that, is that the mapping was arranged so that diesel notch 8 was reached somewhat ahead of GTEL notch 20. This would have recognized that the load control system on the GTELs could sometimes supervene before notch 20 was reached, in which case the throttle handle would be held at a lower notch. This item in the GTEL4500 operating manual (GEJ-1943) is pertinent:
“Notching Guide – Red Band – When throttle handle is advanced, the pointer swings toward or into the red band. If pointer indicates in the red band continuously, throttle handle must be notched back until pointer indicates in the yellow band. No reduction in power results from this action.”
(The notching meter was additional to the conventional electrical load meter.)
It was probably desirable that trailing diesels could reach notch 8 even when the throttle handle had to be moved back from notch 20. Having diesel notch 8 correspond to say notch 15 or thereabouts on the GTEL control would have achieved that.
Power control on the GTELs was done by a potentiometer in the master controller, which supplied a variable voltage to the excitation system (Amplidyne for the first GTEL4500 batch, Type E (or an early precursor thereof) for the GTEL8500 fleet, and unconfirmed, but I think Static for the second GTEL4500 batch). For control of trailing diesels, a straightforward approach (but not the only option) would have been the addition of cams and switches in the master controller to provide the AV, BV, CV and DV governor control signals. Forward, reverse, generator field control, etc., probably could be taken from the appropriate GTEL trainwires without the need for any interfaces. Similarly the GTEL could have provided control voltage for the diesel, unlike the Milwaukee electric + diesel case where there was full separation of the respective auxiliary electrical systems.
The GTELs had the same potential wire dynamic brake control as on Alco-GE diesel-electrics, so they were directly compatible with (some) diesels on that front . Although if field loop dynamic brake control compatibility had also been required, then the brake control potentiometers would have needed to have adequate current capacity.
PneudyneThe GE 16-notch system. Only partial information seems to be available.
The type of short description often given for the GE 16-notch system is typified by this from the ‘Trains’ magazine 1968 December article: ‘Incidentally, the 16-position GE throttle introduced with the U25B does not break the pattern of compatibility. The throttle still has only seven engine speeds above idle, with every other notch merely providing more finely graduated generator field excitation. Eight-position throttle engines M.U.’ing with the GE’s disregard these excitation-only stages.’
I have not found a comprehensive treatment of the topic. The GE Educational Manual ‘General Electric Model U25B Diesel-Electric Locomotive’ (GEJ-3815) certainly helps quite a bit, but it leaves some gaps and does not, for example, contain any main generator or load control curves. What follows is derived and inferred from various sources, and not guaranteed to be correct.
The main objective of the system appears to have been to obtain finer graduation of starting and low-speed tractive effort, rather than finer graduation of running power, than could be provided with eight notches, this being considered desirable for locomotives that had a relatively high power per axle. That same rationale had been used with the GTELs, which were provided with 20-notch controls. One imagines that in the U25B case, the choice of 16 notches was made to provide a system that was readily compatible with the existing 8-notch system.
Accordingly, there were 16 steps of main generator fixed excitation, corresponding to throttle notches ½, 1, 1½, etc, through 8. This provided 16 basic convex and heavily drooped main generator curves, the maximum current point of each chosen to provide appropriate and closely spaced steps of starting tractive effort where they intersected the standstill line.
There were eight engine speeds, set by the customary Woodward PG governor, which also set the engine load point for each speed, and through the action of the load regulator rheostat, applied eight supervening hyperbolic constant-power curves to the 16 basic generator curves.
1st engine speed was used for notches ½, 1 & 1½, 2nd speed for notches 2 & 2½, 3rd speed for notches 3 & 3½, and so on through 7th speed for notches 7 & 7½. 8th speed was used for notch 8 alone.
The net result was that adjacent notches, such as 4 & 4½, shared the same supervening constant power curve, so that over some of the operating speed range they would not have been any different, although at the extremes they would have differed. At the low power end, where load regulator had but little effect in typical GE control systems, the overlap would have been less, and perhaps notches ½, 1, and 1½ were largely distinct across the operating range.
GE must have been satisfied that finer graduation of standstill/low speed tractive effort, but not of running power, was sufficient, and that additional complexity was not warranted.
The 16-notch control was achieved in an interesting way. The throttle handle controlled a 16-step rheostat in the master controller. This set and supplied the exciter battery field voltage not just for the leading unit, but for any trailing U25B units. This voltage, and the requisite current, was relayed to any trailing units via the dynamic brake excitation XB trainwire. Effectively, the motoring excitation current for all units in a GE 16-notch consist was supplied by the leading unit. Insofar as the XB trainwire was not otherwise used during motoring, its use in this role did not create any conflicts.
Of course, trailing GE units needed to 'know' that a GE unit was in the lead. To this end, a spare trainwire, designated SN was used to signal to trailing GE units that there was a GE 16-notch unit in the lead that signalled trailing units to source their excitation from the XB trainwire. The SN trainwire was live whenever the throttle handle was in any position from ½ through 16, and controlled the MR relays on individual units. This relay, when operated, switched excitation source to the XB trainwire.
Engine speed was controlled by the AV, BV, CV and DV trainwires from the throttle handle in the usual way.
If a GE 16-notch unit found itself in a consist headed by a regular 8-notch unit, the SN trainwire would have zero volts on it. Thus it would switch to local resistor ladder control of excitation, in 8 steps, controlled from the four throttle trainwires, AV, BV, CV and DV. That form of motoring excitation control had previously been used by GE on the Export Universals fitted with Cooper-Bessmer engines and three-field control.
The same 16-step rheostat in the master controller was also used to control excitation during dynamic braking, again via the XB trainwire. Also again, excitation current for the trailing units was supplied by the lead unit, with the SN trainwire switching the MR relays to achieve this. The same general form of dynamic braking excitation control had also previously been used on the Export Universals, albeit with continuously variable potentiometer operated by the selector lever.
If a GE unit was trailing a regular unit, lack of a signal on the SN trainwire would cause it to revert to local control of dynamic brake excitation, using its own load regular rheostat controlled from the XB trainwire from a micropositioner. I imagine that that approach recognized that not all dynamic brake potential wire control systems could supply sufficient current for excitation purposes, only enough for control purposes.
Thus the doubling of the notch count apparently was achieved with the use of just one extra trainwire. Presumably the SN wire was an existing spare in the 27-wire bundle, although which one is unknown.
It is conceivable that the later advent of the GE Type E excitation system allowed improvement to the 16-notch control. With Type E, close approximations to constant power hyperbolic curves could be constructed electrically, with the load regulator then serving simply as a trim and protection against abnormal conditions. Thus, each of the 16 notches could have had its own constant power hyperbola. Whether this was actually done I do not know.
What would be useful here would be the finding of the main generator curve sets, covering all notches, say for the U25B and for a later GE model with 16-notch control and Type E excitation. And also perhaps for the Alco C855, which is understood to have had 16-notch control.
OvermodI thought it would be easy to find 'manual' pictures of the connection arrangements for BLW power, or to locate some of Matthew Imbrogno's comments on modernizing the system for shortline and special use. I am sorry to have failed in this so far. I am sure there are people here, or following the Classic Trains forums, who will know; someone might consult the nooks and crannies of the Bakdwin Diesel Zone site or a Wayback version of it to see what's there.
Probably not quite what you are looking for in that it does not address the modernization aspect, but some information on the BLW control and MU system as it was, is available in this book:
Diesel-Electric Locomotive Handbook – Electrical Equipment
George F, McGowan
Simmons-Boardman, 1951.
The chapter on Baldwin (46 pages) provided a description for and schematics of the pneumatic throttle control system, including dynamic braking, based upon the CE100 master controller as typically used on road locomotives.
A good description, with diagrams, a schematic and a main generator curve, of the pneumatic throttle power control system is provided in AIEE paper 47-36 of 1947 January, ‘A 3,000 Horsepower Diesel-Electric Locomotive for the Seaboard Air Line Railway’, by D.R. Staples, Baldwin, T.L. Weybrew and C.A. Atwell, both Westinghouse.
To quote just a little of the detail, throttle line pressure is zero at idle and 7 lbf/in² at power on. Up to 21 lbf/in², it progressively increased excitation to maximum, via the carbonstat, with the engine still at minimum speed. Thereafter, the engine speed was progressively increased to maximum over the range 21 to 60 lbf/in². The load control was of the single point type, acting at full rack to back off the excitation. That was quite different to say the EMD approach, which assigned unique load control points, monotonically increasing, to each throttle notch/engine speed. That the Baldwin engine was of the flat torque curve type probably facilitated the chosen power control arrangement.
That control and MU system, in conjunction with the Woodward UG8 governor, was used by Baldwin’s Belgian associate and licensee, Cockerill-Ougree, on new builds into the 1960s. I think that in the USA, Baldwin might have adopted the Woodward PG governor for some of its late builds, but that is unconfirmed.
Thank you.
This thread alone makes it worthwhile to keep reading the forum threads.
did creep control MU?
blue streak 1did creep control MU?
Since it is individual to the locomotive, there is no particular point in trainlining actuation, other than perhaps to set up creep mode for other locomotives so equipped before they reach a particular bad section of track. This would be an effective waste of a wire in the 27-pin cabling for little if any real-world gain.
I first saw this in action in the mid-Nineties, on KCS in Shreveport, where a couple of switchers that were nearly visually identical were trying to pull an impossible number of heavy empties up a steep approach grade. They were in MU, with the 'creep control' on one audibly operating (the wheels 'ring' or chitter as adhesion is broken and made) and the other not.
"Hump control" (which is very fine output load reduction as cars are progressively released as the cut comes entirely onto the hump) is something different.
Some further background on some elements of the development of the MU common standard was provided by a couple of brief items in ‘Railway Locomotives and Cars’ 1954 October, referring to both Baldwin and Fairbanks Morse locomotives.
The Baldwin item (p.75) referred to developments over the preceding year, and was titled ‘B-L-H Diesel MU With Those of Other Builders’.
Quoting the pertinent part of the item:
>>>>>>>>>>
1,600-HP Locomotives for Operation with Other Makes - During the past year a dynamic braking controller and field loop circuit was developed and units were placed in service equipped for dynamic braking of B-L-H locomotives in multiple with General Motors units. The braking excitation (field loop) circuits were designed to provide equal braking effort on either of the manufacturer's units. The new controller produces a lower braking excitation (field loop current) on all units over the first portion of the braking range. This requires greater movement of the dynamic braking lever and thus a smoother braking control at high speeds. Units also were placed in domestic service that will operate in multiple with General Motors units in braking and motoring and with Alco units in motoring only. In order for these units to multiple with certain locomotives making use of manual transition, the master controller has a transition drum. A transition lever (selector) which rotates the drum controls manual-transition locomotives operating as trailing units in multiple with a Baldwin unit.
The transition lever also controls dynamic braking of the Baldwin unit operating singly or dynamic braking of the Baldwin unit when the lead unit is in multiple operation. An intermediate dynamic braking stop position prevents complete lever swing to maximum braking without some delay to manipulate the catch release as an instantaneous maximum application may cause serious damage.
Presumably that referred to Baldwin units fitted with the alternative Wemco 8-notch electric throttle control, which I think used the XM-781 master controller. A careful interpretation of what was a said was not that there had not been previous use of that system on Baldwin locomotives, but that such would not have had an MU’d dynamic brake control. Thus the 1953 innovation seems to have been Baldwin’s (or more likely Westinghouse’s) development and application of a field-loop controlled dynamic braking circuit that suited the Baldwin electrical equipment, and whose ‘control curve’ could be arranged for reasonably compatible MU operation with EMD units.
Also in 1953 was the apparent addition of a transition control handle to Baldwin version of the Wemco master controller that allowed the control of trailing Alco and EMD units that had manual transition. The fact that Alco units could be controlled in motoring only indicates that with this round of changes, Baldwin had not introduced a compatible potential wire dynamic brake control.
Additional to the above, it was noted that a humping control option had been added for the 1600 hp locomotive. Whether or not that was MU’able was not said, but one could take the lack of clarification as a ‘no’, assuming that MU’d humping controls were not yet generally in use.
The Fairbanks Morse item (pp.77,78) was ‘F-M Improves Its Locomotives’. The last two of a long list of improvements were:
Hump Control. -With this deice, the power of a locomotive can be very gradually reduced to keep the speed constant as cars are cut off in a humping operation. Conversely, it can be used to apply power slowly when starting a train and so minimize wheel slipping.
Dual-Circuit Dynamic-Brake Control.—Units with the field-loop type of dynamic braking control would not MU in dynamic braking with units using General Electric load control, and vice versa. With a dual-circuit control, which can now be furnished, a leading Fairbanks-Morse unit can control the dynamic braking in a trailing unit employing either type of braking.
The second of those supports the notion that F-M was the first to introduce a ‘two-way’ dynamic braking control. Possibly that development was done in conjunction with Westinghouse, although I understand that F-M had some in-house electrical equipment capability.
Regarding the hump control, F-M patented an MU system for this, described in US2908852 of 1959 October13, filed 1956 September 10. It looks as if the MU capability was a later addition to the original, which was probably not MU’d. The MU control system was basically a servomotor-driven rheostat controlled by a variable voltage, 0 to 74 volts.
At some stage, probably later, I understand that AAR trainwire #1 was used for hump control purposes.
PneudyneThe situations where the standard system was interfaced with control/MU systems that were somewhat different or quite different. Some examples: The SP diesel-hydraulic case. Finding the details is a work-in-progress, see: https://cs.trains.com/trn/f/111/t/65687.aspx?page=2#3410266.
Some examples:
The SP diesel-hydraulic case. Finding the details is a work-in-progress, see: https://cs.trains.com/trn/f/111/t/65687.aspx?page=2#3410266.
By way of background, the need for MU compatibility between diesel-hydraulic and diesel-electric locomotives had been called out several years before the DRGW and SP experiments.
In a 1953 ASME paper (53-A-122) by J.S. Newton of Baldwin-Lima-Hamilton, ‘Hydraulic Torque-Converter Transmissions for Locomotives’, the author had provided an 11-point list of desiderata for diesel-hydraulic locomotives intended for freight of road switcher service in the USA, as follows:
(1) The drive should be designed to develop constant horsepower over a minimum speed range of 5 to 1 and 8 to 1 is desired.
(2) The efficiencies at all loads and speeds should be equal to those of an electric transmission.
(3) The power unit should be mounted in the locomotive underframe—not on the trucks.
(4) The design should permit quick and easy removal of individual axles as well as complete trucks.
(5) Trucks should be suitable for high-speed service—swing bolster or the equivalent.
(6) The locomotive with train must negotiate curves of 21 deg.
(7) Multiple operation of units must be possible.
(8) Multiple operation with locomotives having electric transmissions is desirable.
(9) The drive should permit braking equal to the dynamic brake of the diesel-electric drive where braking power is usually greater than pulling power.
(10) Tractive effort must be equal in either direction of motion.
(11) The drive must be arranged for zero torque at the wheels when the locomotive is stationary.
B-L-H would have been well aware of the consequences of being the “odd man out” when it came to MU compatibility in the US domestic market.
Nonetheless, the 1961 K-M prototypes for DRGW and SP were not built to be MU compatible with the US DE fleet. At the time the DRGW and SP locomotives were ordered, the German industry had not yet adopted a common control and MU system for diesel-hydraulic locomotives, and in fact was still just coming off a learning curve when it came to the degree of “fineness” that was actually required for power control. The main customer, DB, was buying only diesel-hydraulic locomotives, so there would have been no need to seriously consider mixed MU operations. Perhaps the German builders thought that such would have been difficult to arrange, and even if it could have been done, was undesirable.
Be that as it may, both DRGW and SP found the lack of MU compatibility to be a drawback, and in 1963 modified their diesel-hydraulic locomotives accordingly. Apparently DRGW developed a two-way interface that worked at trainline level, effectively doing both electric-to-pneumatic (DE to DH) and pneumatic-to-electric (DH to DE) translations. That would have been an interesting device, but I should not have much expectation that any detailed information about it remains available. On the other hand, SP changed the master controllers, installing the same GE KC99 type as used on the GE U25B and as had been specified for the forthcoming production fleet of K-M locomotives. This required just a one-way interface, from the GE/AAR protocol to the K-M pneumatic power and dynamic brake control protocol, the details of which we may reasonably assume will surface sooner or later.
Then there was the Alco DH643. Alco wrote an SAE paper about this, SAE 650420, ‘The Design-Development of a Mainline Diesel Hydraulic Locomotive’, by H. W. Schmidt
The opening statement of that paper was:
‘THE DESIGN AND DEVELOPMENT of a mainline diesel-hydraulic locomotive was undertaken with the potential of greater adhesion, lower possible continuous speed than a diesel-electric for the same horsepower per axle, and lower maintenance on the transmission system.
Design parameters were that:
1. Locomotive must be compatible and operate in all functions in multiple with existing and future domestic diesel-electric locomotives.
2. Locomotive must be maintained and serviced with existing facilities and personnel.
3. Driving controls and arrangement must be such that crew instruction is minimized.
4. Locomotive to use existing expendable parts such as brake shoes, wheels, oil filters, air filters, drawgears, couplers, springs, etc.
5. Locomotive construction and arrangement be as close as possible to existing units to facilities routine servicing.
6. Locomotive to be of road-switcher type commonly used domestically and be of conventional appearance.
7. Locomotive to use a medium-speed, heavy-duty diesel, engine of proven performance and readily available spare parts.
8. Locomotive to be equipped with standard-type running gear suitable to American roadbeds and service.’
MU compatibility had been elevated to numero uno. Unfortunately, the paper did not explain how that was achieved. It did mention that the hydrodynamic brake had 16-step control. But the diagram of the transmission and hydrodynamic brake assembly showed the eight-step (three piston) electropneumatic brake controller, labelled as ‘8-point filling regulator’ Oops! I think that was a stock Voith diagram of the time. There was a later Voith diagram showing the 16-step (four piston) brake controller, and it is known that Voith offered eight-step, 16-step and stepless options.
Absent hard information on the Alco control approach, one is left to speculation. Quite possibly it did use the GE KC99 master controller on the DH643, which as I understand it was also used for the C855. And given the precept of maximum commonality with the existing diesel-electric fleet, I should expect that it would not have departed from the use of its standard 251 engine governors of the time, namely the Woodward PG and the GE MG8. The PG could be configured for up to 15 engine speeds. The MG8, with electrical resistance speed control, had no inherent limit, so probably could have been configured with 15 or 16 speeds.
jeffhergert SD70Dude You can get the MR and trainline gladhands to go together, but you have to twist one hose around 180 degrees, and even then I don't think the gladhands will 'lock' together properly. Our local mechanical trouble shooters have rigged up a hose that has a gladhand that fits the brake pipe on one end, the other end's gladhand will fit the main reservoir hose. They did this so they could recharge the MR on a dead DP equipped with the air start system. This way they don't have to get a live engine next to the dead one to restart it. It comes in quite handy when the mid-train DP has the auto stop shut down, but then doesn't automatically restart and then the MR bleeds down. That happened to me once. I figured we would have to set over half the train to the other main track to get the head end next to the dead DP. The mechanical guy came out, applied the compromise hose and was able to get enough pressure in the main and the auxilary air start reservoir to restart it. The bad habit of DPs shutting themselves down, but not starting up again when needed is why I don't care for the Auto Engine Stop/Start systems. At one time, engines in DP mode weren't supposed to have the AESS systems active because of that problem. It doesn't happen everyday, but way more often than it should. Jeff
SD70Dude You can get the MR and trainline gladhands to go together, but you have to twist one hose around 180 degrees, and even then I don't think the gladhands will 'lock' together properly.
You can get the MR and trainline gladhands to go together, but you have to twist one hose around 180 degrees, and even then I don't think the gladhands will 'lock' together properly.
Our local mechanical trouble shooters have rigged up a hose that has a gladhand that fits the brake pipe on one end, the other end's gladhand will fit the main reservoir hose. They did this so they could recharge the MR on a dead DP equipped with the air start system. This way they don't have to get a live engine next to the dead one to restart it.
It comes in quite handy when the mid-train DP has the auto stop shut down, but then doesn't automatically restart and then the MR bleeds down. That happened to me once. I figured we would have to set over half the train to the other main track to get the head end next to the dead DP. The mechanical guy came out, applied the compromise hose and was able to get enough pressure in the main and the auxilary air start reservoir to restart it.
The bad habit of DPs shutting themselves down, but not starting up again when needed is why I don't care for the Auto Engine Stop/Start systems. At one time, engines in DP mode weren't supposed to have the AESS systems active because of that problem. It doesn't happen everyday, but way more often than it should.
Jeff
There are plenty of instances where you want to get the load off the airdump doors by simply moving the train.
The Saturday NS deailment brings up a question. Is there any way the 27 point MU cable damage could disable some functions of an loco? What is the effect of the MU air hoses going to atmosphere when locos have a wreck separation as what did happened ?
One fundamental problem with MU-cable wreck damage is very similar to a problem at the Brown's Ferry nuclear powerplant.
You may recall this as the plant where not-so-bright design engineers optimized the number of 'penetrations' in the containment by running most of the cabling, including supposedly redundant systems, through one channel, and then more not-so-bright personnel tried checking an air leak in the channel with a candle. The insulation on the cables did not sustain combustion... but it supported it, and the candle flame went back along the channel, burned off the insulation en masse... and all the little wire conductors started touching each other.
At random
With heaven knows what shorts between what conductors.
When an MU cable is pinched in an accident, the conductors may not be severed or attenuated; they may easily be clamped together in such a way that the insulation on the wires is displaced so the conductors come into contact with each other. I can remember at least one accident report, many years ago, where MU cable damage of a specific kind was reported as a causative factor -- someone here will remember what it was.
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