Electrical Insulators

[Bonzo 2000]

Why are the insulators that carry high voltage lines (like power transmission and railway overhead lines) shaped as a series of stacked disks.  In the railway variety and some lower voltage power supply lines, they appear to be made from a single piece of glazed fireclay with the disks cast into the shape.  Other higher voltage lines appear to be a series of glass disks with metal portions between?  Also, why do the disks appear to be ribbed on the underside - I can appreciate that ribbing on the upper side would allow rainwater to collect, but does the ribbing help the overall resistance?  Hope someone can shed some light. Thanks

[MargeB]

Ribbing is probably for structural strength or to stop the rainwater running a course down into the centre and onto conductors??? The current would take the path of least resistance through a 'resistor' and ignore any ribs.

[gazzawazza]

I'm not sure if I'm answering your question from the right angle but here goes.  The larger transmission towers are carrying cables at higher voltages, therefore they need greater distances to prevent the electricity "jumping" or arcing from the cables (which aren't insulated) to the pylons.  This would be very dangerous as the pylons themselves would then become live, and anyone coming into contact with the pylons would be electrocuted.  Now, imagine trying to work with a single, heavy piece of porcelain at a great height.  This might be possible on a building site with cranes and other lifting devices, but imagine trying to get a crane to work offroad up a hill somewhere where many pylons are.

To me it seems the logical answer is to break the insulator up into more easily manageable pieces.  As far as I can remember, each disc can provide insulation for 11000v, therefore a tower carrying 440000v can be identified by it having  40 discs.

Not sure about the ribbing though, I'll have to have a think about that.

[Thunderchild]

yea i think you more or less hit the nail on the head. also having seperate disks will increase the resistance making a better insulator because of at a microscopic level the contact point between disks is not perfect so increasing the resistance