International projectionist (Jan-Dec 1945)

Conduit Changes in The Projection Room By HEISRY B. SELLWOOD WHENEVER projection room apparatus is installed, or when any changes are made that involve alterations in conduit runs, the projectionist should be consulted. The work usually will be done by an electrical contractor with few if any ideas about special requirements of the projection room. He will introduce splices into low-level sound wiring, if nobody stops him. It will never occur to him that points of junction and accessibility should be provided to help in trouble-shooting in a hurry. There are, of course, contractors with experience in projection room work, but they are in the minority. In some locations, where conditions permit, the projectionist may want to do the work himself and be paid for it. In every case he should approve the plans in advance — and he should be paid for studying them, and for his expert knowledge of the projection room. Some of the terminology common to electrical contractors, however, will not be familiar to all projectionists. Figure 1 illustrates the four commonest types of conduit fittings. They are referred to as A fittings, B fittings, and so on, exactly as labeled in that illustration. Figure 2 shows three other of the commonest types, the LB, T and X fittings. But while these are the fittings most commonly used, and the ones that probably will meet ninety percent of all requirements, there are hundreds of others — that fact should be clearly understood. For example, the LB fitting has one conduit connection at the side opposite to the opening. — at the right of the LB illustration. If it were so constructed that the connection were made to a side adjacent to the opening — the side facing the reader — it would be an LR fitting. If it were so arranged that the connecting conduit at the right of the drawing ran straight down through the paper and through the table on which the magazine is resting, it would be an LL fitting. There are also LBB and LRB and LFT fittings and so on, almost without number. Some of these may meet some special requirement better than any one of the more usual fittings illustrated here. If the contractor recommends any such, or if the fittings he does recommend do not seem the most perfectly suited imaginable, the projectionist should ask to see the contractor's catalogue, or obtain one of his own from an electrical supply house. Methods Generally Used There are two general methods of fastening conduit to the fittings, as illustrated in Figure 3. To use the fitting shown at the left of that illustration, a thread is cut on the outside of the conduit, and the conduit is threaded into the fitting. This is the type of fitting shown in Figure 2. Cutting a thread in conduit can be avoided by using the type shown at the right of Figure 3. It represents the same type illustrated in Figure 1. The end of the conduit, unthreaded, is simply slipped into the fitting, and the nut is then made up tight. The conduit is very firmly and permanently gripped. Whether threaded or threadless connections are to be made may depend in part on the type of conduit and fittings already installed. The rectangular openings in the fittings are of course closed with suitable covers, which may be metal or porcelain, and may be solid or pierced with small openings through which wires can be led out from the conduit system. Such A B FIGURE 1 (above) C E FIGURE 2 (below) FIGURE 3 covers may have from one to eight holes, accommodating from one to eight wires. The size of fitting selected depends upon the size of conduit used, and this in turn depends upon the number and size of wires to be drawn through the conduit. Underwriters' specifications govern the size of conduit to be used for every requirement, but it is always well to exceed the required conduit size, despite the additional cost. When wires are drawn through without sufficient clearance insulation may be scraped off. leading to grounding troubles in the future. Also, it may prove necessary, in connection with some future trouble, to draw a wire out of the conduit, and if there was insufficient clearance to begin with that may turn out to be very difficult, and waste a great deal of time during some future emergency. Lastly, oversize conduit will permit additional wires to be drawn in, in the future, if further changes are made, and then those changes may require very little or no additional conduit. For flexible conduit (Greenfield) the no-thread type of fitting (Figure 1 and at the right of Figure 3) is essential unless a flexible-cable connector is added, since Greenfield can't very well be threaded. Other connectors and couplings are also available in wide variety, including "reducers"' which enable any size conduit or fitting to be joined to one of different size. NATIONAL FIRE CODES FOR 1945 OFF THE PRESS "National Fire Codes for Flammable Liquids, Gases, Chemicals and Explosives, 1945," has been issued by the National Fire Protection Association, and appears at a time when war demands are for increasingly larger quantities of these materials, whose fire and explosive hazards involve great danger to human life and property. The book assembles for the convenience of the reader the many standards dealing with these hazards. Superseding the 1943 edition, the new volume contains up-to-date information on new chemicals and solvents used in war industries, including information on their fire hazard properties and the best method of fire extinguishment. In addition there are six new or revised codes. The price of the book is S3 per copy, and may be had from the National Fire Protection Association. 60 Batterymarch Street, Boston, Mass. APRIL 1945 21