Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Replies 11 replies
  • Subscribers 37 subscribers
  • Views 1108 views
  • Users 0 members are here
Options
You may also be interested in

Overload device negate requirement for In<Iz?

E95

Hi all,

For context this is a TP, N & E socket circuit supplying a machine. The machine supplier has recommended that the circuit be supplied from a 125A MCB, however the machining cabling is not supplied from a cable rated to 125A.

I'm being told that a cable size doesn't need to be sufficiently sized for the upstream breaker, when there is an overload unit downstream and that the cabling would withstand a short circuit disconnecting quickly

Is this true? I can't find anything explicit in the regs confirming either way, aside from Ib<In<Iz

Parents
  • 0

    Just wondering about the 125A MCB recommendation from the manufacturer. Seems very blunt with no type stipulation. I wouldn’t have thought a MCB was an appropriate choice for such a situation. MCCB with long time pick up adjustment or maybe MPCB or fuse might be better.

    Having been recently involved in helping in the commissioning of a major installation in a milking parlour, I am not at all confident that manufacturers of some equipment are as technically aware as some might think. I don’t think it is necessarily prudent to blindly defer to their instructions. 

Reply
  • 0

    Just wondering about the 125A MCB recommendation from the manufacturer. Seems very blunt with no type stipulation. I wouldn’t have thought a MCB was an appropriate choice for such a situation. MCCB with long time pick up adjustment or maybe MPCB or fuse might be better.

    Having been recently involved in helping in the commissioning of a major installation in a milking parlour, I am not at all confident that manufacturers of some equipment are as technically aware as some might think. I don’t think it is necessarily prudent to blindly defer to their instructions. 

Children
  • 0 in reply to lyledunn

    Hi Lyle, they actually suggest a "slow blow fuse", however the manufacturers panel includes an overload unit and circuit breaker, which, I guess negates the requirement for this

  • 0 in reply to E95

    It may be that the innocent looking phrase "Slow blow fuse" tells a story. 

    I suspect they are aware of a inrush problems that they are not entirely on top of ;-) Their fix is to specify an oversize supply, a thin and so slightly more resistive cable than normal, and to be a bit vague about speed ratings of fuses so its not their problem. If you have not already installed then be ready to change the breaker to an MCCB with a programmable delay or a maybe 'D' type MCB if there are inrush trip issues.

    Or of course a slow blow fuse ;-)

    Mike

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines