Merry Christmas

[cliffed]

Can anybody tell me,what’s the difference worrying about short circuit conditions on Zs readings that don’t comply,but are protected by a rcbo,difference I’m after are on TN & TT.
The Zs would be still high on a tt system like a tn system,when circuit does not comply, maybe the cable would not comply on short circuit calculations,on both.

 

[Pretty Mouth]

Can anybody tell me,what’s the difference worrying about short circuit conditions on Zs readings that don’t comply,but are protected by a rcbo,difference I’m after are on TN & TT.
The Zs would be still high on a tt system like a tn system,when circuit does not comply, maybe the cable would not comply on short circuit calculations,on both.

Hi cliffed. Please could you clarify your question? When you say short circuit calculations, do you mean the adiabatic equation applied to a L-N fault?

 

[cliffed]

Hi cliffed. Please could you clarify your question? When you say short circuit calculations, do you mean the adiabatic equation applied to a L-N fault?

yes,the thermal constraints on the cabling

 

[SparkyChick]

Are you specifically concerned about a L-N short circuit? If so, Zs is irrelevant, as is the RCD element of the protective device. You need to measure the L-N loop impedance, which even for TT should be low enough for the MCB element to adequately protect the cable. If it's not good enough, then it's the usual method for fixing it... upgrade the cables to drop the loop impedance or drop the RCBO rating so the circuit loop impedance is lower than the maximum Zs for the device.

 

[cliffed]

If the Zs is high the Pscc would be low, so would the cable be @ risk concerning the short circuit fault condition

 

[SparkyChick]

If the Zs is high the Pscc would be low, so would the cable be @ risk concerning the short circuit fault condition

Zs is the earth fault loop impedance so the PEFC will be low, this is where the RCD element comes in, but if you're talking about a L-N short, then you'll need to check the L-N loop impedance which is what governs PSCC. Like I said, I suspect you'll find the L-N loop impedance is low and thus PSCC is high, so the MCB element will work as expected in the event of an L-N short.

 

[cliffed]

Agree, but l-n may not be a low resistance,cables may have the same csa.

 

[Pretty Mouth]

If the Zs is high the Pscc would be low, so would the cable be @ risk concerning the short circuit fault condition

Something to bear in mind is that the PSCC would be highest at the origin of the circuit. The further downsteam of the OCPD the L-N fault is, the greater the impedence, therefore the lower the PSCC.

We use the adiabatic equation to ensure the cables don't fry in the event of a short circuit, ie that the let through energy (I²t) is low enough. For BS60898 MBCs, the let through energy increases as fault current increases (see below attachment). Worst case is at the origin of the circuit, so use the PSCC from here for your calculation.
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