Type of MCB .........................

[Toneyz]

In domestic when the Zs easily low enough why do we default to have type B instead of C curve MCB's/RCBO's

 

[westward10]

There is no line of separation between a so called domestic protective device and a device for other purposes, it is mindset.

 

[bill01803]

Back along with wire wound transformers d type was the one to use unless to completely dropped the loading of the circuit.
Don’t you remember walking into a a room and hearing the transformer start!

 

[Andy78]

There is no line of separation between a so called domestic protective device and a device for other purposes, it is mindset.

Are the different product standards not a line of separation ?

 

[westward10]

Are the different product standards not a line of separation ?

The product standard is EN60898 or EN61009 it is the type characteristics which define the intent.

 

[Andy78]

The product standard is EN60898 or EN61009 it is the type characteristics which define the intent.

I was meaning the difference between 60898 and 60947-2, but I think I may be getting a little lost in the argument.
My point was that if a manufacturer's 60898 offerings are solely B curve, that could be a reason why it has become mindset to install them as default.

 

[Lucien Nunes]

An MCB has two main functions:

a) To protect cables and accessories from overheating through overload. This is done by the thermal tripping mechanism of the MCB and follows the same curve for all types. Provided IB<In<Iz all is well and the cable will always be protected, because the characteristics of the cable and MCB are known and matched.

b) To protect cables and accessories from overheating, limit the dissipation of energy at a fault, and limit the duration of shock due to high touch-voltage, by disconnecting faults rapidly. This is done by the magnetic tripping mechanism, and the current threshold at which this takes over from the thermal mechanism depends on the MCB type. In effect, the value of I/In defines the point at which the MCB switches purpose from keeping the cables within ratings to clearing a possibly life-threatening shock fault or imminent fireball ASAP.

In a perfect world, whenever a fault occured, it would be a solid short-circuit of zero resistance that would dissipate no energy and allow a current of Uo/Zs to flow, hence trip an MCB of any permissible type. But in reality, faults are random in nature. Energy is often dissipated in destructive arcing and heating, and the current may not be as high as the Zs would suggest. Therefore, a curve that transitions to instantaneous tripping at a lower I/In may trip faster, reducing exposure to touch voltage and limiting energy dissipation at the fault. Under these conditions, a B-type may offer greater safety, provided it is compatible with the load, which for a mixed domestic load it usually will be.

 

[westward10]

An mcb has one function, to disconnect due to excess current by way of fault current or over current. The sole function is always an excess of current.

 

[Lucien Nunes]

This is just a matter of semantics. There are two fundamentally different scenarios in which an MCB might react to 'excess current' that demand different sensitivity and speed. But in both cases, it is the excess current that triggers a tripping mechanism inside the MCB.

 

[Bobster]

The product standard is EN60898 or EN61009 it is the type characteristics which define the intent.

MCB's that are built only to the standard EN60898 should not be used in an industrial environment.

Not really conducive to this thread but I felt I should point it out.

It's not a common issue as most MCBs are built to meet the requirments of both EN60898 and EN 60947-2 that latter being the requirement for industrial applications.

 

[l4urence]

The only time I've needed to fit a type C was for a welding machine, pretty inductive loads can look like a momentary short on start up and cause it to trip. This was dedicated 16A supply, be wary that the max Zs values change (almost a half lower) for the whole circuit - from 2.73 to 1.37 Ohms. Be mindful of this when using for other connections (additional 13A sockets on the same circuit).

 

[TJ Anderson]

In any case, as already stated, ADS needs to be achieved whatever type you install.

Also, more of an issue, there are a lot of 61009 RCBO's in houses that are C type curve. Uo*cmin/Zs not high providing enough PEFC for the MCB part so then relying on the RCD part for of the unit for ADS under earth fault, this is ok.....but L-N shorts are covered by the MCB part only and mostly this isn't even being checked! (T=(K2*S2)/I2). Although often fine as cpc reduced compared to live conductors CSA, Calc should be insisted on if ADS under earth fault is only being achieved by RCD part of the RCBO.