I wonder if I'm the only one here who has experienced exactly this episode
on the same model Hobart trolleybus! The conductor very quickly got the
poles back on the lines.

Tony P

On Thursday, 24 May 2018 12:07:52 UTC+10, efftech wrote:
>

> Geoffrey's question < Do trolleybuses also need switchable overhead

> frogs?> was replied by Chas from the perspective of current European

> practice, but the situation was quite different during the years when

> trolleybus systems operated in Australia, i.e until the late 1960s.

>

> In Hobart & Launceston the setting of overhead frogs was under the control

> of trolleybus drivers via a product known by its trade name 'Selectric

> Frog'. This employed sensors placed on both wires ahead of a turnout which

> were actuated by the passage of the trolley pole heads.

>

> The operation of these sensors, whether simultaneous or staggered,

> controlled the setting of the frogs. To achieve staggered operation of the

> sensors, the driver had to manoeuvre his vehicle at an angle to the line

> of

> overhead prior to reaching the turnout thus causing the two trolley pole

> heads to actuate the sensors sequentially, as opposed to simultaneously if

> he had proceeded dead ahead.

>

> However not all drivers under all conditions, i.e darkness, proximity of

> motor vehicle traffic, etc, achieved the angle necessary to achieve

> sufficiently staggered operation of the sensors in which case the circuit

> controlling the setting of the frog did not respond. The consequence of

> this

> was that the trolley bus followed the route steered by the driver and the

> poles followed the other route, leading to an inevitable dewirement.

>

> Hobart trolley bus drivers were sometimes able to discern such an

> impending

> dewirement, pull up in time, and reset the poles manually. This evasive

> action was possible due to the Hobart vehicles all being equipped with

> retrievers. Unless obscured by standee passengers, drivers could observe

> the

> attitude of the retriever ropes in their rear vision mirrors, and became

> adept at recognising whether the poles were heading in the wrong

> direction.

> Of course their ability was reduced after darkness but by that time

> traffic

> density had diminished, allowing them to execute a more exaggerated angle

> to

> ensure staggered operation of the overhead sensors.

>

> An example of the type of dewirement described in the foregoing is

> depicted

> in the attached screenshot frames extracted from a some 8mm film.

>

> Hobart No 234 is on a short working service to Queenborough where it will

> follow a turning circle and return to the city. The driver has positioned

> it

> at an angle prior to the poles reaching the overhead frogs but not enough

> to

> achieve the necessary amount of stagger to switch the frogs to the turning

> circle overhead.

>

> The bus heads into the turning circle but the poles continue straight

> ahead

> for Sandy Bay. The driver has picked this up in his rear vision mirror,

> stopped his vehicle, reset the poles and proceeded round the turning

> circle,

> thus avoiding a dewirement and the potential damage that may ensue.

>

> This sequence of events can be followed in the 5 screen shots.

>

> Clearly, the Selectric frogs did not offer the same 'fail-safe' operation

> assured by the Czech system referred to by Chas.

>

> Roger Greenwood

>

>

>

>

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