Published:
11 August 2009
Category:
Q&A
Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning a 25A BS88 type fuse and Zs values, is answered by SELECT:
Question: I'm currently reviewing electrical test certificates carried out in 2006. The maximum Zs value shown for a 25A BS88 type fuse is written down as 1.91 ohms. I cannot locate this in the 16th Edition that was current at that time.
Do you have any idea where this value has come from? I am assuming that it perhaps came from an older version of the 16th Edition. Do you have anything you can send me that has old Zs values for previous 16th Editions?
Answer: The maximum Zs values for various protective devices in the 16th Edition were given in Tables 41B1, 41B2, 41C and 41D. These are for conductors at their maximum operating temperature (eg: 70oC for PVC).
Do you have any idea where this value has come from? I am assuming that it perhaps came from an older version of the 16th Edition. Do you have anything you can send me that has old Zs values for previous 16th Editions?
Answer: The maximum Zs values for various protective devices in the 16th Edition were given in Tables 41B1, 41B2, 41C and 41D. These are for conductors at their maximum operating temperature (eg: 70oC for PVC).
The IEE On-Site Guide, however, gives maximum measured values under normal testing conditions. The value given in 16th Edition versions of the On-Site Guide for a 25A BS88 fuse for a five second disconnection time with a circuit protective conductor of between 2.5mm2 and 16mm2 is 1.92 ohms.
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