Help On 17Th Edition Regs Question

[amateursparky]

Hi

I'm stuck on this particular question. Our tutor hasn't taught us how to solve this question yet.

Could you please explain what it is I need to do to solve this question. I didn't think I even understand what the question is asking!

1)An ac circuit is protected by a 20A BS 60898 Type C circuit-breaker and supplementary bonding may be required. In relation to the accessible exposed conductive parts of this circuit and extraneous conductive parts what would be the maximum resistance that would determine the requirement for supplementary equipotential bonding?

2)An ac circuit is protected by a 40A BS 88-3 system E and G fuse and supplementary bonding may be required for a

disconnection time of 0.4s. In relation to the accessible exposed conductive parts of this circuit and extraneous conductive parts what would be the maximum resistance that would determine the requirement for supplementary equipotential bonding?

My tutor said something about using the fuse graphs at the back and the formula on pg66 (green 17th edition). Very confused!! I don't understand how to use that formula either HELP

 

[NozSpark]

Ok then I'll tackle the first one which should then give you some pointers as to how to do the second... BTW this is off my head!

So the circuit has a C20 BS60898 MCB and from BS7671 (BGB) we know that a 20A final circuit should have a disconnection time of 40mS (oops - should have said 0.4s) or less. For this to happen we need a certain current to travel around the L-E loop and this can be determined by looking at the fuse/current graphs in the back of your BGB..... Once you have this current value you can then use ohms law to calculate the maximum allowed resistance for the circuit before you need to redesign the circuit or add a supplementary CPC

HTH

 

[kerching]

Ok then I'll tackle the first one which should then give you some pointers as to how to do the second... BTW this is off my head!

So the circuit has a C20 BS60898 MCB and from BS7671 (BGB) we know that a 20A final circuit should have a disconnection time of 40mS or less. For this to happen we need a certain current to travel around the L-E loop and this can be determined by looking at the fuse/current graphs in the back of your BGB..... Once you have this current value you can then use ohms law to calculate the maximum allowed resistance for the circuit before you need to redesign the circuit or add a supplementary CPC

HTH

Mr Merlot concurs with your hypothesis

and I think you are right as well :coat

 

[NozSpark]

My post now edited, should be 0.4s not 40ms :?

 

[amateursparky]

thanks

 

[Nicky Tesla]

Just find the max Zs . if your circuit is above this, then add supp bonding until your R1R2 is low enough to bring the Zs under the max

 

[the doctor]

Hi All,

Is the first question not related back to section 411? ( havent got book to hand, but around page 50 ) where the resistance of a supplementary bond is stated as

R= 50/I so the voltage between two bits of connected metal is limited to 50V. For the 20A type C then the figure would be 50/ 200 = 0.25 ohm

 

[the doctor]

Hi again

Q2 is similar..... Find out how many Amps you need for a 0.4 disconnection. Either do that from the graphs in appendix 3 or get the maximum Zs figure from chapter 41 and divide that into 230 V to get the current ( I=V/Z)

Again, divide that current into 50V to give you the maximum resistance of the bonding conductor.

 

[widdler]

Hi All,

Is the first question not related back to section 411? ( havent got book to hand, but around page 50 ) where the resistance of a supplementary bond is stated as

R= 50/I so the voltage between two bits of connected metal is limited to 50V. For the 20A type C then the figure would be 50/ 200 = 0.25 ohm

I'd have to agree with that, it appears to be what the question is asking for.

Should the resistance value between to accessible conductive parts exceed 0.25 ohms then the touch voltage present would exceed 50 Volts and of course this is not acceptable, therefore supplementary bonding would need to be installed.

 

[Chr!s]

Hi

I'm stuck on this particular question. Our tutor hasn't taught us how to solve this question yet.

Could you please explain what it is I need to do to solve this question. I didn't think I even understand what the question is asking!

Very poor questions

1)An ac circuit is protected by a 20A BS 60898 Type C circuit-breaker and supplementary bonding may be required. In relation to the accessible exposed conductive parts of this circuit and extraneous conductive parts what would be the maximum resistance that would determine the requirement for supplementary equipotential bonding?

Is Supplementary Bonding required becuase of the location of the install or because disconnection time's can not be met? Let's assume disconnection time can not be met, this means the Impedance of the Earth Fault Loop is to high and not enough current will flow to disconnect the CPD in the required time(We dont know the type of system to know what the required disconnection time is). you can find the various Max Zs in the BGB Chapter 41.

So we now have decided to install Supplementaty Bonding, to ensure a touch voltage between teh various conductive parts, 415.2.2 tells us R<50 v/Ia for ac. So Again we can use the Max Zs values to determine Ia ot use the table to the side of the Time Current curves in Appendix 3, using the above formula we can acheive a max resistance for our Supplementary Bonding Conductor.

2)An ac circuit is protected by a 40A BS 88-3 system E and G fuse and supplementary bonding may be required for a

disconnection time of 0.4s.

This seems to sugest using Supplementary Bonding to lower the Zs of the circuit, this is in correct, we do not use Supplementary Bonding to lower Zs, its to place a limit on touch voltage.(Installing supplementary Bonding may bring the circuit Zs within the required value)

In relation to the accessible exposed conductive parts of this circuit and extraneous conductive parts what would be the maximum resistance that would determine the requirement for supplementary equipotential bonding?

This again relates to 415.2.

My tutor said something about using the fuse graphs at the back and the formula on pg66 (green 17th edition). Very confused!! I don't understand how to use that formula either HELP

Its no wonder your confused, they arnt great questions HTH