Cable length limit question

[Wattsy]

Evening all

Hopeully someone can help me get my head around this.

I'm looking at table 7.1 OSG. For the 1361/3036 45A radials on a TN-S system the limiting factor is sc, which means its limited by the line to neutral loop impedance (short circuit). But the TNC-S it isn't.

Can somebody please explain this to me in detail please i'm confused:C

Many Thanks

 

[bluetobits]

Bit of a puzzler that. I would have thought Zs was more of a limiting factor due to less current flowing than a SC fault to break fuse link.

 

[Wattsy]

Its a puzzler alright.

Off to work now hopfully someone will have shed some light on the situation by the time i get home

 

[sparkytim]

As the circuit is RCD protected (Zs limit 1666 ohms) then limited by L- N loop impedance for Short circuit fault.

I wonder how many actually measure L-N loop iompedence?

 

[TonyCE]

Following the units... in 6mm2 cable, @ 7.3mV/A/m (which is 0.0073ohms per m)

Max resistance as per Chris above is 230V/240A (240 A is from fig 3.1 pg 244 BRB) = 0.96Ohm

thus length of cable that will allow resistance of (0.96 - 0.8) = 0.16Ohms

0.16Ohm/0.0073Ohms/m = 21.9m

 

[sparkytim]

Tables are based on worst case so TNS Ze taken as 0.8 ohms but TNCS 0.35 ohms

 

[bluetobits]

Still looking at table 7.1 OSG, If i were to install a 2.5 cable on a 16A RCBO on a TN-S system, how is the 27 meter figure calculated?

230V/160A = 1.43ohms-0.8 = 0.63/.018 = 35meters :C

 

[bluetobits]

Still looking at table 7.1 OSG, If i were to install a 2.5 cable on a 16A RCBO on a TN-S system, how is the 27 meter figure calculated?230V/160A = 1.43-0.8 = 0.63/.018 = 35meter

Is this the 16a type c 27zs?

Yes that's the one

 

[bluetobits]

well this is referring to the Zsso Assuming .8 max =.63/ .0235 = 27m

Where does .0235 come from? Is it tabulated or did you just reverse the equation?

 

[Chr!s]

Where does .0235 come from? Is it tabulated or did you just reverse the equation?

Where did you arrive at the .018

 

[Wattsy]

The table are there so you don't have to carry out design calculations.Its assuming a tns has a l -n loop of .8 ohms

A 45 radial(table 7.1) , 5 sec disconnection requires 240 amps to flow, so 230/240= 0.96

16mm t/e 58sc(meters) x 2.8/1000 = .16 add the .8(tncs) and you have your 0.96.

I thought the table assumes Ze (L - E loop) of 0.8 ohms not L - N loop?

How is the L -N loop different on a tnc-s and tn-s?

 

[bluetobits]

Where did you arrive at the .018

Table 6E2 OSG voltage drop

 

[Chr!s]

I thought the table assumes Ze (L - E loop) of 0.8 not L - N loop?How is the L -N loop different on a tnc-s and tn-s?

Well i suggest its using the measurement for both, the table is there for non calculation.

They have to arrive at a figure which wont compromise safety.

If you design the circuit yourself then you will work with the actual parameters for that installation.

 

[Chr!s]

Table 6E2 OSG voltage drop

27zs this is your line cpc resistance