Solutions to Zs too high

[gman67]

Hi,



Hi all,

I am doing the 2391 course at the moment and I'd like to know the best solutions to a high Zs value of a circuit which does not meet BS7671 requirements. This high reading doesn't come from any damage or loose connection but due to a design mistake. The circuit is protected by a MCB type B.

I suggest:

• to replace the MCB with a RCBO (it's apparently acceptable but not always the best solution)
• To replace the MCB with one type C
• To install supplementary protective bonding conductors
• To replace conductors with ones with higher CSA (too expensive and time consuming)

Please tell what would be the best option in terms of good practice but also cost effectivity

Thanks

 

[mattmoo1974]

Smaller MCB?
Facts and figures are needed )

 

[gman67]

Smaller MCB?
Facts and figures are needed )

Hi mattmoo,

I don't have any figure, it's just an invented scenario, I am just trying to figure out the best remedial work when we measure a zs which does not meet the requirement in bs7671 but not coming from any deterioration on the circuit.

let's say its a radial circuit with a 20A mcb type B

 

[Sintra]

You definately don't change a B type MCB to a C type if the Zs is allready too high for a B type.

 

[telectrix]

as above. a C type would make it worse. drop from 20A to 16A if the load is under 16A.

 

[mattmoo1974]

you need to tell us a scenario.. the suggestions u offered up are the real word...
you wont pass 2391 with.. To replace conductors with ones with higher CSA (too expensive and time consuming)

well maybe, But without the last bit ^_^

 

[scotsparky]

If you can change the type
If its just over stick an rcbo on.
If its for example 5 ohms over i would suspect a loose terminal so start hunting

 

[topquark]

You may be able to split the circuit into two or more smaller ones, use RCD/RCBO, run a separate CPC (of larger diameter). You would problably know if you had a termination/connection problem (based on the length of cable you'd run).

 

[TPES]

Easiest and simplest answer is provide 30mA RCD protection, this is what they will expect to see on your 2391 as an answer, as adding this will then make the max Zs limit 1667 ohms, if your answering the question on paper you could always mention a second choice if possible would be to bring the resistance in the circuit down by installing a greater CSA cable, ( as you suggested) this will help bring your loop reading down to an acceptable limit.

 

[gman67]

Ok, thanks so if I understood, best options would be either to replace the MCB with a RCBO or run a separate cpc (or higher CSA).

Thanks TPES and Topquark for your straightforward answer!

 

[topquark]

As TPES said they'll be looking for you to suggest the RCD/RCBO route as the prefered option.

 

[brucelee]

additional protection by 30mA rcd
Increase csa of conductors
change cb from type c to b but not b to c as others have already kindly pointed out
shorten lenght of run but highly unlikely this will be possible unless you move connected equipment also
1st or 2nd option the best

 

[Rampage]

Isn't the maximum Zs of 1667ohms on TT only?

 

[Guest123]

Isn't the maximum Zs of 1667ohms on TT only?

The 1667ohms max figure applies to a 30mA RCD, regardless of where it is. This only ensures that the device will operate & clear an earth fault before the STV of 50V is exceeded.

50/0.03 = 1666.67

 

[ezzzekiel]

phone the dno and tell them that the max Ze of 0.35 is just too high could they please come and reduce it this morning - thanks.