To detect conditions and to forestall events in which an electron or positron beam might otherwise melt a hole in its SLC arc vacuum chamber, and to provide information on the magnitude and location of these and other, less harmful, beam losses, it is planned to install two PLIC cables along each arc, one on each side of the magnet string. A similar system (but with one long ion chamber) has been in use at SLAC for 20 years. Electromagnetic shower calculations have been made to estimate the ion chamber signals which will be produced in the new system, making use of data from the old system. It is estimated that a potentially damaging beam loss in an SLC arc will give rise to an ion chamber signal pulse with an amplitude on the order of 2 to 5 volts. Depending upon the location of a beam loss, sensitivity may vary from its average value by as much as 40%. Most of this signal is due to charged particles in the electromagnetic shower that are able to escape the magnetic field and reach PLIC.