mcb question

Mr Mark Sparks · Jul 26, 2012

why cant you put a 32a type c mcb on a domestic 2.5/1.5 ring main if the earth is tn-s ???

Jimmy Boy · Jul 26, 2012

Why can't you ? Where does it say that ?
J

MKS · Jul 26, 2012

As long as zs et al are all good, don't foresee a problem

Mr Mark Sparks · Jul 26, 2012

subby on site brought eca book in because i didnt belive him ( NP not permited )

somersetsparks · Jul 26, 2012

I honestly didn't know that! I suppose the max Zs of 0.72 (100%) is a real limiting factor, only giving you 0.58 to play with in reality?

keniff · Jul 26, 2012

That is for a type c breaker don't know if that's anything to do with it

keniff · Jul 26, 2012

keithsims said:
That is for a type c breaker don't know if that's anything to do with it

Didn't read first post properly sorry lol

MarkieSparkie · Jul 27, 2012

Hi Mark, If you look at the title of the ECA page you refereed to, you will notice it's a design table for Standard Circuits (similar to those in the OSG 7.1 Tables 7.1). These easy design tables, by their simple nature make worst case assumptions. In the instance you highlight, for a 32A Type C MCB protection device for a RFC on a TN-S system, one of the assumptions made is that external earth loop fault impedance Ze is at the maximum quoted by the DNO ie. 0.8Ω. If you now look at BS7671:2008+Amt 1:2011 Table 41.3 the maximum design Zs for the 32A Type C MCB is 0.72Ω, therefore the assumed Ze>Zs max and cannot comply, and therefore is Not Permitted (NP) as shown in the ECA table.

somersetsparks · Jul 27, 2012

MarkieSparkie said:
Hi Mark, If you look at the title of the ECA page you refereed to, you will notice it's a design table for Standard Circuits (similar to those in the OSG 7.1 Tables 7.1). These easy design tables, by their simple nature make worst case assumptions. In the instance you highlight, for a 32A Type C MCB protection device for a RFC on a TN-S system, one of the assumptions made is that external earth loop fault impedance Ze is at the maximum quoted by the DNO ie. 0.8Ω. If you now look at BS7671:2008+Amt 1:2011 Table 41.3 the maximum design Zs for the 32A Type C MCB is 0.72Ω, therefore the assumed Ze>Zs max and cannot comply, and therefore is Not Permitted (NP) as shown in the ECA table.

So ia ssume if you come accross it on a PIR and it tests fine the its not worth mentioning? Most TNS supplies I see now are not true TNS, they are usually jointed to PME mains somewhere nearby, thus giving very good readings.

MarkieSparkie · Jul 27, 2012

If you were to test and inspect such a final circuit and it's Zs <(0.8x0.72Ω)=0.576Ω, applying the 80% Rule of Thumb to compensate for testing at ambient temperature, then it would comply with BS7671:2008+Amt 1:2011 EFLI requirement.
Obviously, the example referred to assumes the RFC is NOT additionally protected by an RCD to BS7671:2008+Amt 1:2011, 415.1.1. Where such RCD additional protection is used the ELFI requirement (BS7671:2008+Amt 1:2011, Table 41.5) would be much less onerous, irrespective of the primary protective device or earthing system used.

dim_bulb · Jul 27, 2012

What reason would you need to is it because the ICn 10ka as a possed to a 6ka on a type B?

MarkieSparkie · Jul 28, 2012

dim_bulb said:
What reason would you need to is it because the ICn 10ka as a possed to a 6ka on a type B?

No, it's nothing to do with the Icn rating.
The usual reason for choosing to use Type C MCB in preference to a Type B would be in situations where switching surges would be likely to operate a Type B MCB. Typical situations occur in commercial and industrial installations where there is a wide spread use of discharge lighting and small motors. For example the highly inductive nature of fluorescent lighting banks is a particularly common source of switching surges requiring a less sensitive MCB.
Type B MCBs typically operate in the conventional time (0.1 to 5s) at a trip current between 3 and 5 times the nominal current rating (In) of the device. Whereas Type C MCBs are less sensitive and typically operate at a trip current between 5 to 10 In.

Guest123 · Jul 28, 2012

A little bit misleading is that table IMO, as it seems to suggest the actual final circuit is not permitted where as its actually that particular protective device that doesn't comply due to the reasons mentioned above.

It would be much clearer if they simply stated that a 32A C type MCB would not comply on a TN-S system where the external Ze value exceeded the maximum permitted Zs for the device.