what exactly is "flashover" damage please?

[tadhatter]

recently picked up a old video (yes that old!!!) on the class 50s and the the narrator kept commenting on the problems they had with flashover damage on the LSW route. However, it never explains what flashover damage actually was/is!

Can someone please enlighten me?

 

[westcoaster]

Only thing I could find http://nzetc.victoria.ac.nz/tm/scholarly/tei-Gov01_05Rail-t1-body-d23.html

...Flash-overs occur on main generators, motors, etc. A flash-over is the dissipation of a large amount of electrical energy from the positive point to the negative or earth. In the case of the main generator this energy flashes in the form of a large are from the positive sets of brushes to the negative sets, or the earthed iron work of the machine, through the intervening air space...

 

[CatfordCat]

recently picked up a old video (yes that old!!!) on the class 50s and the the narrator kept commenting on the problems they had with flashover damage on the LSW route. However, it never explains what flashover damage actually was/is!

Can someone please enlighten me?

I have found a fairly simple explanation

A flashover occurs when the brushes in the commutators of DC traction motors touch each other because of a unusual movement or bump, such as when a locomotive goes over a grade crossing. If the brushes touch each other, an electrical short can occur almost like a lightning strike. Obviously, this can severely damage the traction motors. Locomotives with DC traction motors must have the throttle reduced slightly when going over a grade crossing to reduce the risk of a flashover. Newer locomotives with AC traction motors have no commutator brushes and therefore do not have to be throttled down at grade crossings.

from http://exotic.railfan.net/dieselfaq.htm

I have read somewhere that the 50s didn't like the relatively stop-start nature of the stopping trains on the Exeter - Waterloo service, having been intended for longer distance work. I don't know how accurate that is - I don't see that it would have caused the kind of 'bump' referred to (unless the track was very rough in places)

 

[Rich McLean]

Happens with 43s as well, especially when not increasing power in stages (ie, Notch 1 to Notch 5 in one go). With the Amps high while going over crossovers is high risk. Often when there are issues with TMs, on power cars, notches are often taped out. . So if Notches 4 and 5 are taped out, the engine will only rev up to notch 3, which reduced the risk and protects the Traction Motors

 

[John Webb]

A flashover occurs when the brushes in the commutators of DC traction motors touch each other because of a unusual movement or bump, such as when a locomotive goes over a grade crossing. If the brushes touch each other, an electrical short can occur almost like a lightning strike. Obviously, this can severely damage the traction motors. Locomotives with DC traction motors must have the throttle reduced slightly when going over a grade crossing to reduce the risk of a flashover. Newer locomotives with AC traction motors have no commutator brushes and therefore do not have to be throttled down at grade crossings.

I'm not certain this is the correct explanation, because the brush holders in a DC traction motor are firmly fixed relative to each other, usually on opposite sides of the commutator, and therefore there is no way in which the brushes can 'touch each other'!

I have always understood 'flashover' to refer to an excessive voltage which exceeds the design of the motor wiring insulation and therefore causes damage. This can occur in a number of ways - bear in mind that any inductive circuit, when suddenly broken, will generate a large voltage as the magnetic field collapses and if there is no means of dissipating this voltage through a circuit, the voltage can be much higher than that which set up the magnetic field in the first place. A very simple demonstration of this is in the humble battery-operated door-bell; if you look at the contact that opens as the bell operates, there is a distinct spark as the contact opens.

 

[michael769]

I have always understood 'flashover' to refer to an excessive voltage which exceeds the design of the motor wiring insulation and therefore causes damage.

There are a number of suspected causes for flashover - and this is one.

Another is where conductive contaminants build up low conductivity tracks that reduce the air gap between brushes. These contaminants can either arise from carbon wearing off the brushes or from "copper drag" which is where copper from the commutator rings gets deformed and worn off.

Vibration can cause the brushes to come too close and short in some motor designs (known as brush jumps).

http://www2.iee.or.jp/ver2/honbu/14-magazine/log/2009/2009pdf-d/2009_11d_06.pdf

 

[broadgage]

Most unlikely to be the brushes touching each other, as posted above they are well fixed and a significant distance apart.

Flashover is generaly understood to be serious or sustained electrical arcing around the commutator, likely causes are over voltage as might be caused by switching surges or very sudden changes in load, or contamination of the comutator surface by conductive dirt.
Carbon dust from brush wear, coal dust, or soot from engine exhausts are likely causes.

I have worked on large DC generators and motors (outside of rail industry) and great importance was attached to keeping commutators clean.
Some machines had arrangements for cleaning the comutator by compressed air without stopping the machine.

 

[W230]

Flashover is generaly understood to be serious or sustained electrical arcing around the commutator, likely causes are over voltage as might be caused by switching surges or very sudden changes in load, or contamination of the comutator surface by conductive dirt.
Carbon dust from brush wear, coal dust, or soot from engine exhausts are likely causes.

The best description so far I think!

I always thought it was basically severe arcing or even a brief short circuit, which as you highlight, could be caused by a variety of reasons.

 

[Pacerpilot]

It happens on class 66s under extended 75mph running from my experience. It will kill the power for a short while, then, once the Gen. Fault lamp goes out, you can take power again.

 

[Whistler40145]

Does anyone know why 50026 had a flashover at Keighley?


I expect a flashover can occur e.g. a locomotive attempting to pull a heavier load than is designed & during adhesion problems?

At what point does a Main Generator flashover?

 

[tadhatter]

thanks for all this. I got the impression from the video (althuogh it didn't actually say so) that flashovers were a problem with running on 3rd rail electrified lines. That might be pure speculation on my part though.

 

[455driver]

The problem with the 50s on the Southern is the way they were driven, because of the high Current generated they needed the power to be reduced gradually but Southern men were used to going from full power to off in one quick movement which, if the generator was a bit long in the tooth, could cause the genny to flash over.

The main cause of flash overs is the electricity is lazy and will take the easiest route to earth and if that is along the dirt along the insulation rather than through the windings then the high currents caused a very loud bang.

 

[Peter Mugridge]

I boarded a CRP once at Waterloo East for Charing Cross and it did repeated flashovers all the way - very loud bangs from underneath, the whole carriage noticably jumping upwards each time.

Reported it to a senior bod upon arrival at Charing Cross and got told "they'll sort it next time it's in the depot"; I wonder if there was anything left of the specific traction motor in question before it got even a quarter the way back down the line - it was let straight back out in service on the booked departure without even being looked at...

 

[TOCDriver]

If anybody has ever watched the original film, 'The taking of Pelham 1,2,3' I think you'll find the classic flashover is when she 'bucks' . But from the very little knowledge i have, they occur in DC motors when the brushes touch each other due to irregular movement or whatever. Newer trains with AC motors do not have brushes and therefore do not have flashovers.

 

[millemille]

Newer trains with AC motors do not have brushes and therefore do not have flashovers.

Flashovers can occur anywhere where a potential difference exists between supply and earth a short circuit can occur.

AC traction motors are not immune from this - a phase can flash over to earth and does.

Flashover is a generic description for any occasion where a +ve supply short circuits to earth. Pantographs can flashover, VCB's can flashover, shoe gear can flash over etc etc.

 

[Wyvern]

You may get some arcing at the contact between the brushes and the commutator. THis ionises the air around it which forms a low resistance path. If this gets drawn round by the rotation of commuataor it may facilitate a flashover.

 

[Murph]

Flashovers can occur anywhere where a potential difference exists between supply and earth a short circuit can occur.

AC traction motors are not immune from this - a phase can flash over to earth and does.

Flashover is a generic description for any occasion where a +ve supply short circuits to earth. Pantographs can flashover, VCB's can flashover, shoe gear can flash over etc etc.

I believe it doesn't have to involve shorting to earth, but rather can be a short between any 2 potential levels where there's sufficient difference. E.g. between 2 phases of AC, phase and neutral, phase and earth, positive and negative DC (without negative necessarily being tied to earth), etc.

 

[455driver]

I believe it doesn't have to involve shorting to earth, but rather can be a short between any 2 potential levels where there's sufficient difference. E.g. between 2 phases of AC, phase and neutral, phase and earth, positive and negative DC (without negative necessarily being tied to earth), etc.

Very true but as some people are obsessed with the fact that a flashover only happens when brushes touch each other (have they actually seen how far apart the brushes are in a TM or generator?) then things need to be kept simple.

A bolt of lightening is effectively a flashover.

 

[TOCDriver]

Very true but as some people are obsessed with the fact that a flashover only happens when brushes touch each other (have they actually seen how far apart the brushes are in a TM or generator?) then things need to be kept simple.

A bolt of lightening is effectively a flashover.

I'll email Network Rail about this because I'm not convinced. Will send you a copy of their response

 

[swt_passenger]

Speaking as an electrical engineer, I'd go with the straightforward definition RAIB uses:

Flashover

An unwanted electrical discharge between two or more items of
equipment usually causing damage.

I searched for 'flashover' on the RAIB site, it brings up three reports here, none are to do with brush gear:

http://www.raib.gov.uk/search_resul...cl_lngurlcode=&lcl_othrlngsite=home/index.cfm

The idea that the term is confined to brushgear faults is very simplistic.

 

[ainsworth74]

I'll email Network Rail about this because I'm not convinced. Will send you a copy of their response

Why would Network Rail be able to comment about a rolling stock issue? They're the infrastructure operator...

 

[TOCDriver]

Why would Network Rail be able to comment about a rolling stock issue? They're the infrastructure operator...

They do have electrical engineers and electricians you know Who do you think install and look after the wires? What we're discussing is an electrical / physics issue. NR have more than enough expertise in that area.

 

[ainsworth74]

I suppose but I'd have thought someone qualified/experienced in working on electric motors rather than the electricification systems would have been of more use.

Also an electrical engineer has already commented, see above

 

[swt_passenger]

Another example of the use of 'flashover' to describe a fault (not in a motor or generator) occurs in the fairly recent RAIB report about the defective linebreaker in the fire on the TW Metro here:

http://www.raib.gov.uk/cms_resources.cfm?file=/130925_R182013_South_Gosforth.pdf

 

[TOCDriver]

I suppose but I'd have thought someone qualified/experienced in working on electric motors rather than the electricification systems would have been of more use.

Also an electrical engineer has already commented, see above

I know, but it's electricity at the end of the day; NR engineers have also got to understand how traction operates as well you know

 

[Teaboy1]

The term 'Flashover' is a generic term and could mean several things but it is loosely a condition where the insulation falls to such a low level that the voltage will [like water] always seek the path of least resistance. It makes no difference if it is AC or DC, its still electrikery .... and it ALWAYS seeks the path of least resistance.
The insulation is generally compromised by contamination, pollution or mechanical damage and whether its between adjacent phases or earth, when the insulation breaks down, it allows a small current to flow. Over time it increases and damages the insulation allowing the situation to worsen. In cases of 'partial discharge' PD to engineers, it can be across the surface of an insulator [due to contamination] or via voids or internal fissures within the composition of the insulation. The surface PD is a slow gradual process that can take weeks / months to lead to failure and is generally seen as a fern-like or dentifrice-sintilation or dentritic-sintilation. In sub-stations or indoor switch-rooms, you generally know or sense something is wrong when you first enter as you can smell the ozone this process produces, so its wise to vacate PDQ!
The case of DC brush gear flash-over would be caused again by some pollution or moisture usually as a result of deposits of carbon dust building up where it should not.

If there is a weak point or path in / across / through / over / around / under / tunneling through any form of insulation [solid, gaseous -air- or liquid] then the voltage will eventually find it out. Never forget water / moisture is the worst enemy of electrickery except demineralised water that is a pretty good insulator right up to 22kV in modern water & hydrogen cooled alternators.

Its a massive subject and would fill several hard drives !!

 

[455driver]

I'll email Network Rail about this because I'm not convinced. Will send you a copy of their response

Please do, I could do with a laugh!

How many NR electricians work on locomotive main generators again?
Is it more than zero, I have forgotten.

Maybe I should ask Steve, he is a domestic electrician and I am sure he will be as well qualified to comment on the subject as any NR electrician!

 

[HSTEd]

Don't national rail still have some locomotives on their own account?
Do they not have the maintenance staff for them?

 

[TOCDriver]

Please do, I could do with a laugh!

How many NR electricians work on locomotive main generators again?
Is it more than zero, I have forgotten.

Maybe I should ask Steve, he is a domestic electrician and I am sure he will be as well qualified to comment on the subject as any NR electrician!

You're being a bit unfair there in my opinion. I know an engineer who works for them with a 1st class electrical engineering degree from Imperial college. He's in a rather different league than your domestic electrician in all honesty. Like I say, physics is physics, no matter where that physics is happening - be it in generators or simple plugs. I wouldn't scoff on topics I don't know much about, 455 driver - it makes you look rather arrogant.

 

[455driver]

I wouldn't scoff on topics I don't know much about, 455 driver - it makes you look rather arrogant.

I was referring to those that believe a flashover can only occur on DC generators and then only when the brushes touch each other which is just wrong on all counts.

I wonder what happened to the AC alternator on the rear powercar on 1C81 today, as it has an alternator it cant poosibly have had a flashover can it!

As for appearing arrogant I would probably find it hard to care less now, the cowmanure that I have read about my job and TOC that some people post as fact is truely amazing at times, of course that stuff is allowed to stay but curt replies are deleted as 'confrontational' or 'off topic' or the favourite of the day 'a tidy'