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“Flash-Overs”
What They Are, Their Cause and Effect

(By William H. King, Shift Engineer, Power Station, N.Z.R., Otira)

Now that the Railway Department in New Zealand has taken on a new phase in railroad engineering, namely heavy electric traction, a new term has become common, and that is “flash-overs.” No railway system is perfect and trains will be delayed at times. The electrification scheme at Otira also has its delays. When a train stops passengers ask, “What's the trouble?” and some one in the know says, “Oh, they've had a ‘flash-over.”’ Then someone probably asks, “What's a lash-over?”' As often happens, the person interrogated may have but a hazy notion of the import of the phrase he has used and his expianation may make it more hazy still to the party seeking information.

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. The generators at Otira are 1,500 volts and have an output of 1,200 Kilowatts at full load rating. The brush sets are separated from each other by about 18 inches and the nearest earthed point is about 12 inches. When a flash-over takes place this separating air space is broken down through the carbon gas off the brushes being blown across by the windage of the commutator and forming a path of low resistance for the electric current to follow. The electric are in dissipating the air is very rapid and the noise made is like the roar of a cannon but of a much sharper nature.

To suppress this destructive brush are a special device is fitted to the generator circuit breaker, which in its operation causes the field switch of the generator to be opened. Of course this does not stop the flash instantly, as the fields are magnetically saturated and it takes time for them to lose their magnetism. I have seen a brush are suppressed in the time taken for one half revolution of the commutator. Since the armature revolves at 450 revolutions per minute the duration of flush will be one-fifteenth of a second.

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A light flash of short duration merely blackens the commutator and deposits on its face a few pips of copper. The latter are easily removed and the black marks are cleaned off with sandpaper. When it has been ascertained that all is correct on the overhead line, the machine is then ready to be run up to speed and put into service once more.

A severe flash-over will last several revolutions of the commutator and in its path will burn the brush arm insulation even to mica 1–16 in. thick. The flash guards covering up the brush sets will have holes burned in their inside corners of such a size that one's finger could be put through them. The flash guards are made of cast iron and fit snugly over the sets of brushes, so that the burning takes place on the cast iron guards and not on the expensive brush rigging.

A flash-over is the outcome of a dead short circuit or nearly so, which in other terms is a path of extremely low resistance to the flow of electric current. In the case under discussion it is caused by a rise in current or amperes, much in excess of the full load rating of the machine.

One of the worst flash-overs is caused by a dead short circuit near the power station such as the contacts wire, catenary or suspension wire being pushed against an earthed girder, or some other earthed suspension. The term “earthed” means being connected by a heavy copper conductor to the track which, in its turn, is connected to the negative terminal of the generator.

However, to finish up the explanation, it may be stated that a flash-over, like the one under discussion, will be in the region of 20 times the output of the generator, which is a machine of 1,200 Kilo-watts full load output. Thus the amount of energy thrown into the the flash will be 24,000 Kilo-watts or about 32,000 horse power for the duration of flashover. This large amount of power is derived from the momentum of the revolving armature which weighs about 7 tons, and also the turbine motor with its speed of 3,000 revolutions per minute.

When some of these brush arcs occur there are small pieces of carbon in a white hot state thrown off the brushes by the commutator, and these incandescent particles dance on the floor some 8 or 10 feet away for a couple of seconds or so.

Another item worthy of notice is the fact that no flash-overs occur due to the overhead being earthed at Arthur's Pass end, but just a heavy overload which is taken care of by the circuit breakers. The reason for no flashover is due to the overhead line resistance and also the return track resistance. No doubt when a large increase of current is demanded suddenly, reaction plays its part in a favourable way, and with a highly magnetic return such as the steel track it must be of considerable value, and thus help to choke bock the heavy rush of current.

Without going into reasons and technical details, etc., it is worth stating that rotary converters are more prone to flash-overs than straight out generators and not only flash-overs on dead short circuits, but also flash when the A.C. supply fails.

One of the latest developments in alternating current conversion to direct current is the mercury are rectifier. When it flashes over no noise is heard, but just simply the circuit breakers tripping. This is satisfactory to the page 40 nerves of the power station operative, but the linesman on the outside mains gets the brunt of it, for it is his business that suffers considerably.

Before concluding it is perhaps worth mentioning that railway companies when ordering alternators for traction work sometimes specify that they are to be able to stand up to short circuits without detriment to the machine. This class of machine is vastly removed from the direct current class, as they have neither commutator nor revolving contacts on the power output side and the current is not unidirectional as in the D.C. machine, but as its name implies alternating and, in British practice, goes through 50 complete cycles per second. The design is of such a nature that when subjected to short circuits the inherent reaction of the machine windings on the short circuit current makes the circuit breakers trip the machine off load before any appreciable damage is done. This feat in machine design, threw extra attention on to circuit breaker design also.

The generators at Otira were tested in the Dick Kerr works at Preston to the following loadings and times.

100% overload for 5 minutes.

150% overload for 30 minutes.

125% overload for 2 hours.

As compared with other places we have had about our share of flash-overs at this power station. In the early days we had as many as four in a single day, but on the other hand, there have been periods of from six weeks to two months between flash-overs.

[To prevent flash-overs the modern practice is to install high-speed current breakers which disconnect the line sufficiently quickly to avoid any flash-over on the machine.—Ed., “N.Z.R.M.”]

The American railroads have recently been authorised by Congress to carry any totally blind person accompanied by a guide at the fare charged for one person, under such reasonable regulations as may have been established by the carrier. It may not be generally known that this has been the practice on the New Zealand Railways for many years.