Outbuilding

[Traineeboy]

Still learning during this lockdown. 

Just a question in regards to testing if you are on the live set of tests and testing at a garage board. When you measure the ZE and remove the incoming earth from the garage DB (because of parallel paths )I presume you keep the earth connected in the main consumer unit ? As I am only measuring the ZE at the garage unit DB. 

I obviously then know with my result I add my my ZE to my R1 + R2 readings to get me ZS reading.

 

[Murdoch]

Still learning during this lockdown. 

Just a question in regards to testing if you are on the live set of tests and testing at a garage board. When you measure the ZE and remove the incoming earth from the garage DB (because of parallel paths )I presume you keep the earth connected in the main consumer unit ? As I am only measuring the ZE at the garage unit DB. 

I obviously then know with my result I add my my ZE to my R1 + R2 readings to get me ZS reading.

why would you disconnect the incoming earth at the garage dB?

 Surely that would mean there is no earth and remember your tests of the new sub main give you zdb at the garage cu

 

[Traineeboy]

Ok just to confirm. 
 

if I was measuring Ze at the main CU I would remove the main incoming earth due to parallel paths but at the garage CU as it’s not a Ze I would be measuring ,it would be Zdb I wouldn’t remove any earths ? I just thought that if I left the earth in the garage unit it would then pick up parallel paths from the main CU via the gas and water bonding ? 

 

[Murdoch]

Ok just to confirm. 
 

if I was measuring Ze at the main CU I would remove the main incoming earth due to parallel paths but at the garage CU as it’s not a Ze I would be measuring ,it would be Zdb I wouldn’t remove any earths ? I just thought that if I left the earth in the garage unit it would then pick up parallel paths from the main CU via the gas and water bonding ? 

Put it this way, would you disconnect the main earth if you were testing a new immersion circuit - answer no. 

Would you disconnect the main earth to test any final circuit / submain?

I'm assuming the garage isn't TT BTW

 

[Traineeboy]

Right got ya. I assume if the garage CU was connected via Henley blocks (spilt tails) then it would be a Ze at the garage board. 
 

 

[Murdoch]

Right got ya. I assume if the garage CU was connected via Henley blocks (spilt tails) then it would be a Ze at the garage board. 
 

what is the earthing arrangement at the out building?

 

[Traineeboy]

It will use earth from the CPC on the SWA which is connected to the a main CU which is earthed via a TN-S system. 
 

but was asking if the garage unit was connected via Henley spilt tails ( main earthing system TN-S) then the measurement at garage unit would be Ze not Zdb . Is that right ? 

 

[Andy™]

It will use earth from the CPC on the SWA which is connected to the a main CU which is earthed via a TN-S system. 
 

but was asking if the garage unit was connected via Henley spilt tails ( main earthing system TN-S) then the measurement at garage unit would be Ze not Zdb . Is that right ? 

Ze is external to the installation. so if your installation starts at the henley blocks then since the shed DB is at the end of your cable then youll be measuring Zs of the submain, which would also be Zdb, not Ze

 

[Evans Electric]

Measuring in feet suggests a very mature trainee.

Not even I measure in feet  Murdo .  

 

[Traineeboy]

Ze is external to the installation. so if your installation starts at the henley blocks then since the shed DB is at the end of your cable then youll be measuring Zs of the submain, which would also be Zdb, not Ze

ok thanks . So if I was calculating Zs of say a lighting circuit off the Garage DB. I would measure the Ze at a Henley block/isolation switch. The R1 and R2 of the lighting circuit. 
 

would it be Zs = Ze ( R1 + R2) Ze measured at Henley block or Isolation 

or would it be Zs = Zdb ( R1 + R2) Zdb measured at the garage CU main switch. 

 

[Murdoch]

ok thanks . So if I was calculating Zs of say a lighting circuit off the Garage DB. I would measure the Ze at a Henley block/isolation switch. The R1 and R2 of the lighting circuit. 
 

would it be Zs = Ze ( R1 + R2) Ze measured at Henley block or Isolation 

or would it be Zs = Zdb ( R1 + R2) Zdb measured at the garage CU main switch. 

or:

Ze + (R1 + R2 for the submain) + (R1 + R2 for the circuit in question)

If that makes sense

Watch your volt drop too - especially on long submains 

 

[Traineeboy]

thanks to all who take the time to reply. Another thing I just want to clarify. 
 

If say you have 15 led downlights on one circuit and they are equivalent to 50w a light. So 750W divide by 230 = 3.2 A 

But the 6A circuit breaker keeps tripping am I right in thinking as long as there isn’t a fault then it’s due to inrush current. This can then be rectified by fitting a type C MCB rather than a type B ? 

 

[Phoenix]

The 50w is irrevelant, thats the manufacturer saying "This light is as bright as a 50W tungston halogen", do your calaculation using the actual wattage of the fitting, though I'd still use the 1.8 fiddle factor, even though that might be a bit different for LED, its a good starting point. And us going to a breaker with a higher instantous tripping threadhold is one answer, subject to Zs constraints and also thermal withstand of the CPC (unlikely to be an issue here, but its one that I bring up when someone wants to fit a D type to a lighting cirucit in a commercial premises wiried in T/E). Other ways are don't have all the inrush at the same time; switch the lights in smaller banks, then you will have separate peaks of inrush rather than one big one.

 

[Murdoch]

thanks to all who take the time to reply. Another thing I just want to clarify. 
 

If say you have 15 led downlights on one circuit and they are equivalent to 50w a light. So 750W divide by 230 = 3.2 A 

But the 6A circuit breaker keeps tripping am I right in thinking as long as there isn’t a fault then it’s due to inrush current. This can then be rectified by fitting a type C MCB rather than a type B ? 

As long as the 15 LED's are on 1 switch in rush is unlikely to be an issue

Changing to a typc c will have consequences on your calculations for Zs - so tread with care

 

[Traineeboy]

Thanks . 
 

so it’s better they are on 1 switch than multiple switches ?  
 

If I ever did need to switch to a type C then I guess it’s just knowing the different Maximum permitted Zs of a type c MCB to a type B found is BS7671. 
 

on that subject am I right in thinking that on-site guide is 0.8 of the readings in BS7671. So does that mean that if a reading is 0.9 of the reading in BS7671 it’s a fail or needs to be investigated ? 

 

[Murdoch]

Thanks . 
 

so it’s better they are on 1 switch than multiple switches ?  
 

If I ever did need to switch to a type C then I guess it’s just knowing the different Maximum permitted Zs of a type c MCB to a type B found is BS7671. 
 

on that subject am I right in thinking that on-site guide is 0.8 of the readings in BS7671. So does that mean that if a reading is 0.9 of the reading in BS7671 it’s a fail or needs to be investigated ? 

my post has a typo

 never put that many led lights on the same switch

 

[Traineeboy]

I assumed so but was just checking 

 

[binky]

Thanks . 
 

so it’s better they are on 1 switch than multiple switches ?  
 

 15 downlights - sounds like a large area y0u are illuminating? I would be inclined to group sets of downlights to give different lighting effects,  eg in a kitchen, I would separate an island unit / under cupboard lighting / and general area lighting so the customer can have soft lighting / full on bright lighting depending on what they are doing.

As for in-rush currents, these are in the order of micro-seconds and should not be picked up be a breaker type B or not. in my opinion, tripping MCB is more to do with the quality of downlighter used.

 

[Murdoch]

As for in-rush currents, these are in the order of micro-seconds and should not be picked up be a breaker type B or not. in my opinion, tripping MCB is more to do with the quality of downlighter used.

some manufacturers do have recommendations for the maximum per circuit

 

[binky]

some manufacturers do have recommendations for the maximum per circuit

not noticed that, but I still recon quality of components is main issue.