measured Zs to high

[ohthatshowyoudo]

Hi all
please can someone assist me i have measured Zs at the end of a lighting circuit Zs 1.48 Ω and the mcb is 1.44Ω so it fails. But checking appendix 14 the equation... Zs (m) < 0.8 x Uo / Ia = 0.632 Ω passes. Am i missing something please can you advice new at this need all help i can get.

 

[nikmet]

What size mcb is it??

 

[nickblake]

If it is a lighting circuit on a BSEN60898 6 amp MCB then the max Zs in BS7671 should be 7.66 ohms not 1.44 ,1.44 ohms is for a BSEN60898 32amp MCB

 

[Deleted member 9648]

If it is a lighting circuit on a BSEN60898 6 amp MCB then the max Zs in BS7671 should be 7.66 ohms not 1.44 ,1.44 ohms is for a BSEN60898 32amp MCB

More likely a 16a/C which is also 1.44 ohms.......Unfortunately we cant check your calculation without knowing the type of protective device....but assuming a 16a/C


230/Ia(160)=1.44

0.8 or 80% of 1.44= 1.15
Therefore your maximum measured Zs should not exceed 1.15 ohms.

 

[Deleted member 26818]

It looks like either a 32A type B, or a 16A type C.
Either way, the calculation is the same: 230V divided by 160A = 1.4375Ω X 0.8 = 1.15Ω.
How you managed to get 0.632Ω is anyone's guess?

 

[ian.settle1]

It looks like either a 32A type B, or a 16A type C.
Either way, the calculation is the same: 230V divided by 160A = 1.4375Ω X 0.8 = 1.15Ω.
How you managed to get 0.632Ω is anyone's guess?

Reading the way he has written the formula he has multiplied Uo by 0.8 then divded it by Ia which if you transpose it you get an Ia of 291A

 

[ohthatshowyoudo]

the mcb is type c 1.44 my measured Zs is 1.48 in appendix 14 bs7671 it gives the formula for high measured earth loop which is
ZS (m) < 0.8 x Uo / Ia. i used 291amps

The Ia Was taken from the Uo / Zs = If (Is this were i am going wrong in the equation ?)

but if i use the 160 amps i get 1.150 ohms smack on does this still pass.


cheers for your help

 

[Deleted member 9648]

Ia is the actual current required to operate the protective device within the specified disconnection time.....look at the time/current curves in app 3 and you will see that a 16a/c 60898 mcb reqires a fault current of 160a to operated instantaneously.
Use this value as Ia.......from the formula already worked out above the maximum measured Zs for a 16a/c type mcb will be 1.15 ohms.(this value is simply 80% of the maximum values given in tables 41.2>,which is a rule of thumb figure to allow for increase in resistance caused by load current/fault current which will not be taken into account when testing).
As you have measured 1.48 you will not meet the required disconnection time.You will either have to use a different protective device which will operate at your measured Zs....or find a way of lowering the measured Zs...(larger conductors?)

 

[Deleted member 9648]

Put another way.....we know from app 3 that a 16/C mcb requires a fault current of at least 160a for instantaneous operation........with your measured Zs of 1.48 ohms the earth fault current will be 230/1.48=155a.

 

[Deleted member 26818]

The 160A is the value shown in Fig. 3.5 in BS7671 for the amperage required to cause instantaneuos operation of a 16A type C MCB/RCBO.
Any higher value is irrelevant, as it will not cause the device to operate any quicker.
The maximum measured value of Zs should be no greater than 1.15Ω.
As your measured value is higher than this value, no it does not pass.
Either change the device to one where the measured value of Zs will be acceptable, or adapt the circuit to reduce the measured value of ZS.
Either install supplementary bonding, or re-wire with larger conductors.

 

[johnboy6083]

Tripping currents​

BS 3871 (OLD)​

Type 1 2.7 to 4.0In
Type 2 4.0 to 7.0In
Type 3 7.0 to 10.0In

Type 4 10.0 to 50.0In​

BS EN 60898 (NEW)​

Type B 3 to 5In

Type C 5 to 10In​

Type D 10 to 20In

Note: IEC 898 permits the upper limit for type D to
extend to 50 x In.​

To work out your max Zs, you use the following formula.

voltage/tripping current= max possible ZS (bs7671 tabulated values)

mulitply this by 0.8 to give you values corrected for running temperature

unless the ambient temperature is 10 degrees c then this will aslo need correcting futher, but i dont have my OSG to hand
​