Disconnection times help

[Paul1985]

Hi all just a little bit of advice at the company i work has just had a condition report done and they have marked down excessive zs for a 63a type d breaker but they have put it at 0.4 disconnection time it is a permanent fixed compressor I thought this would have been 5 sec it also has a b.s. 88 63a isolator to help with discrimination the zs is 21 ohms and the supply is a TN S I thought all final circuits above 32a could come under 5 sec as long as it's not special location or has things changed again

Sorry if this is in the wrong place just realised you have a special page for testing

Thanks for any advice.

 

[Richard Burns]

Final circuits over 32A on a TN earthing system have a 5s disconnection time to give you 0.35Ω and 0.68Ω for BSEN60898 type D and BS88-3 respectively as maximum EFLI at maximum operating temperature.
If your Zs is 21Ω then there is something significantly wrong for a TNS system, if it is 0.21Ω then you are OK.

 

[Toneyz]

Are you sure regarding the Zs being 21 ohms?
A distribution circuit and a circuit exceeding 32A for a TN system disconnection time not to exceed 5 s.
Max Zs for a D MCB is 0.35 ohms.

 

[loplug]

even at 5s 21 ohms wont comply

 

[Paul1985]

Sorry type error 0.21 ohms. So I'm right in thinking the circuit is fine and should be calculated at 5 sec disconnection not the 0.4sec they have reported and it is actually there report that is incorrect. On the report it say 0.4 disconnection and a max zs of 0.18ohms a give the report a c2

 

[Toneyz]

0.21 ohms is well under the o.35 max so from that respect it complies to give a disconnection time max of 5 sec. The 0.18ohms is from the info provided incorrect therefore complies and requires no code.

 

[Paul1985]

Cheers for all your help that's what I thought but before going back to them and saying this is wrong the circuit is fine and complies. I didn't wanna make my self look like an idiot and I had missed something. I just wondered if something changed in 3rd amendments and I could find it. Thanks again

 

[Deleted member 26818]

Whilst the disconnection time may be 5s, are you sure the 0.18 ohms is low enough for a D type breaker when allowing for temp and C min?

 

[Paul1985]

Hi people I'm back so my company have spoke to the test and inspect company and the are still saying max zs allowed is 0.18ohms because the table in the regs state this now. Now I know you can calculate this (just struggling doing it at the moment any help appreciated) Iv looked at Schneider tec spec for the breaker and they say the max zs for 5 sec with there breaker is 0.47 16ka.

Thanks again

 

[Spoon]

Hi people I'm back so my company have spoke to the test and inspect company and the are still saying max zs allowed is 0.18ohms because the table in the regs state this now. Now I know you can calculate this (just struggling doing it at the moment any help appreciated) Iv looked at Schneider tec spec for the breaker and they say the max zs for 5 sec with there breaker is 0.47 16ka.

Thanks again

Am I missing something? For a 63A type D breaker with 5 secs of disconnection time is 0.35 ohms, according to Table 41.3. Where do they get 0.18 from.

 

[Richard Burns]

Calculation for maximum Zs is:
at maximum permitted operating temperature
Zs <= (Uo x Cmin) / Ia = 218.5/Ia
at 10°C conductor temperature (accepted correction factor of 0.8)
Zs <= 0.8 x (Uo x Cmin) / Ia = 174.8/Ia
at 20°C (common ambient temperature)
Zs <= (0.8 x (Uo x Cmin) / Ia) x 1.04 = 181.8/Ia

this is assuming you have a 230V Uo, since the circuit breaker is presumably to BSEN60947-2 you would need to reference the manufacturers data to determine the minimum current causing operation of the circuit breaker within 5s to use as Ia. BS7671 does not provide data for BSEN60947-2 so there would be no table to reference in the regulations.
If the BSEN60898 does apply then I have attached the Certsure pocket guide for ambient temperature values for Zs (as adapted from BS7671 AMD3).
The value given for 0.4s operation in BS7671 is 0.17Ω not 0.18Ω.

 

[Wilko]

Am I missing something? For a 63A type D breaker with 5 secs of disconnection time is 0.35 ohms, according to Table 41.3. Where do they get 0.18 from.

Me too
It's either 0.35 or 0.17 Ohms. Since it's over 32A 411.3.2.3 says up to 5 sec is ok, so it's a temperature adjusted 0.35 Ohms that's needed. Not sure if 0.8 is the correct factor to use here (learning), but that gives 0.28 Ohms and measured value is 0.21 Ohms, so all is ok (?).

 

[Paul1985]

Cheers for all your help don t know if it will get me anywhere. See the problem is I'm the company's only spark and no one else understands. We basically have periodic test every 4 years and what ever gets marked down I have to sort (with in reason) this compressor has been In about 12 years before my time there. I thought I might of been missing something with 3rd amendments . I just realised I got one of those pocket guide's somewhere at home. Looks like I'll be telling work o can't fix something that isn't broke and give them the reference out of the regs and table and leave it up to them. They probably dont know what there doing. Like I said thank you all for your help.