[Illustration Fig. 206.  Arrester Separators]

Dust Between Carbons:—­Discharges between the carbon blocks tend to throw off particles of carbon from them.  The separation between the blocks being small—­from .005 to .015 inch—­the carbon particles may lodge in the air-gap, on the edges of the separator, or otherwise, so as to leave a conducting path between the two blocks.  Slight moisture on the separator may help to collect this dust, thus placing a ground on that wire of the line.  This ground may be of very high resistance, but is probably one of many such—­one at each arrester connected to the line.  In special forms of carbon arresters an attempt has been made to limit this danger of grounding by the deposit of carbon dust.  The object of the U-shaped separator of Fig. 206 is to enable the arrester to be mounted so that this opening in the separator is downward, in the hope that loosened carbon particles may fall out of the space between the blocks.  The deposit of carbon on the inside edges of the U-shaped separator often is so fine and clings so tightly as not to fall out.  The separator projects beyond the blocks so as to avoid the collection of carbon on the outer edges.

Commercial Types:—­Fig. 207 is a commercial form of the arrangement shown in Fig. 205 and is one of the many forms made by the American Electric Fuse Company.  Line wires are attached to outside binding posts shown in the figure and the ground wire to the metal binding post at the front.  The carbon blocks with their separator slide between clips and a ground plate.  The air-gap is determined by the thickness of the separator between the carbon blocks.

[Illustration:  Fig. 207.  Carbon Block Arrester]

[Illustration:  Fig. 208 Roberts “Self-Cleaning” Arrester]

The Roberts carbon arrester is designed with particular reference to the disposal of carbon dust and is termed self-cleaning for that reason.  The arrangement of carbons and dielectric in this device is shown in Fig. 208; mica is cemented to the line carbon and is large enough to provide a projecting margin all around.  The spark gap is not uniform over the entire surface of the block but is made wedge-shaped by grinding away the line carbon as shown.  It is claimed that a continuous arcing fills the wedge-shaped chamber with heated air or gas, converting the whole of the space into a field of low resistance to ground, and that this gas in expanding drives out every particle of carbon that may be thrown off.  It seems obvious that the wedge-shaped space offers greater freedom for carbon dust to fall out than in the case of the parallel arrangement of the block faces.

An outdoor arrester for metallic circuits, designed by F.B.  Cook, is shown in Fig. 209.  The device is adapted to mount on a pole or elsewhere and to be covered by a protecting cap.  The carbons are large and are separated by a special compound intended to assist the self-cleaning feature.  The three carbons being grouped together as a unit, the device has the ability to care for discharges from one terminal to either of the others direct, without having to pass through two gaps.  In this particular, the arrangement is the same as that of Fig. 204.