Abstract :
There are at present two distinct methods of rating in use in Great Britain for ordinary-duty fusible cut-outs. The first method is that given in B.S.S. No. 88 and is based on an absolute value of minimum blowing current difficult to determine. The second method, that specified in the I.E.E. Wiring Regulations, adopts the ¿1-minute blowing current¿ as a basis. The former method is generally admitted to be unsatisfactory from a practical standpoint, and the latter method is shown herein to be unsuitable for semi-enclosed cut-outs employing tinned copper fuse-links. The present report reviews British, Continental and American practice, deduces a method of rating suitable for existing British designs, and provides the experimental data required for a revised specification. The problems confronting a designer in calibrating a range of cut-outs to comply with the revised basis are dealt with in detail, together with the simple tests by which the user may check the rating in practice. The information contained herein will be of service in connection with the revision of B.S.S. No. 88 which is now being carried out; but in the interim before a new specification is issued it is thought that a useful purpose will be served by subjecting the proposals to the criticism of designers and users.
