Magnetic field shielding effectiveness of magnaflex conduit in external direct current magnetic fields

The shielding effectiveness of flexible metal conduit when it is subjected to external DC fields is considered. Measurements have shown that the shielding effectiveness of this type of conduit is reduced when a longitudinal DC field is applied. The tests have also shown that, when flexible conduit is used to shield a cable against EMI (electromagnetic interference), the lower value of shielding effectiveness of flexible conduit can cause changes in how the 010 Handbook is implemented. When DC fields of up to 20 G are applied to a cable shielded with flexible conduit, the worst possible net effect would be to increase the susceptibility of that cable by 15 dB. Since each susceptor group number is spaced 20 dB from the next, and the maximum reduction of shielding effectiveness due to a DC field is 15 dB, the net effect will be at worst case to increase the susceptibility of the cable and conduit by one group number (e.g. an S4-6 could be pushed to S4-5)