electric tripproblem

[JohnL]

Just out of interest, As you say you do not like wired fuses, You don,t explain why, but having done some development work on a product where over current protection was important, wired and cartridge fuses could often with stand up to 2 times the rated current for several seconds before they disconnected. By comparison MCB's were much faster, and acted much closer to the stated current limit.

The reason for these issues is that fuse wire relies on its electrical resistance to generate enough heat within the wire to actually melt the wire. It takes a considerable time for the heat to build up in the fuse link due to the mass of the wire, and ambient temperature.

Where as the MCB's use the magnetic field a current produces in a coil. The field strength is a factor of the current flowing. This is used to counter a permanent magnets attraction and a spring will open the contacts. The field can be established in one cycle of mains supply (20mS) though the mechanics of the switch may damped to take longer to disconnect to avoid nuisance tripping, but trip times of less than a second are quite normal.

 

[gary]

I've robbed this MCB explanation as it says it better than I, however my point was and this explains the magnetic field side is secondary to it's thermal protection....

The first characteristic is the overload which is intended to prevent the accidental overloading of the cable in a no fault situation. The speed of the MCB tripping will vary with the degree of the overload. This is usually achieved by the use of a thermal device in the MCB. The second characteristic is the magnetic fault protection, which is intended to operate when the fault reaches a predetermined level and to trip the MCB within one tenth of a second. The level of this magnetic trip gives the MCB its type characteristic as follows: -

 

[gary]

And these graphs show the designed in overload tolerance of MCB's according to their rating.

http://www.buildingdesign.co.uk/elec-technical/hager-t1/hager-miniature-circuit-breakers.htm

 

[ProfJohnL]

Thank you Gary,

I was slightly economical with the truth, the MCBs I was was working with were specialist breakers for use inside a machine, where sensitivity to current overload was fairly critical. The currents could spike hence the fast tripping.

The domestic type as you describe are still quicker acting than most cartridge or wire links.

 

[xtrailman]

john l

what is your source showing the CB reacts quicker than a cartridge fuse?

 

[ProfJohnL]

Hi Ray,

I have no source to show, only the memories of results of practical test work carried out for a employers.