Cb rating [ka]

[jimbo123]

I have read about this same question on here but I am not sure how to answer this because the first thing he will ask me is ' show me in the book '.

We are told when ever we do any electrical work like [ pm ]on equipment that has a circuit breaker rating above say the note 1 at the end of the task tables in70e [ such as 25ka scc available: with a max. of 0.03sec clearing time [2 cycle ] fault clearing time ]. They are saying we can not work there because it has a ka rating above 25ka.

How is work performed using the tables if you need to do a study to see if the requirements are below the values in the notes?
Where do you get the clearing times from ?
Will this be explained in 70e ?

I hope this came across ok because i am confused. Thanks for the help..

 

[zog]

You got it, you need to know the available fault currents and clearing times in order to use the tables. So some engineering needs to be done.

You can get an idea of those values by looking the the OCPD's AIC rating (Your availbale fault current is assumed to be less than that) and your clearing times by knowing what type of trips your breakers have. Usually, if you havbe INST trips on your breakers or have fuse protection you should meet the clearing time limits.

 

[kingpb]

The only way to properly know is by calculation. The energy released during an arc fault can vary greatly depending on fault current and tripping/clearing time. IMO: the only assumption you can make, is that without the calcs, the PPE level required is greater than 4, and therefore you cannot safely work hot.

Also, keep in mind that both IEEE 1584 and NFPA 70E only consider radial systems. They do not address mesh or loop systems, or systems with multiple sources.

BTW: the level of PPE only helps to determine that a life threatening injury should not occur. Which means, severe injury is still a very real possibility.

 

[jimbo123]

King are you saying a backup gen or solar panels and you could not use 70e?