cable resistance

[OGNIB]

Hi ,I have asked 2 electricans had 2 diferent ansewers can anybody help, my inverter works when the resistance ,at the inverter end ofthe cable,is less than 1 ohm, is that value the resistance of just the AC cable between inverter and CU, or the value of AC cable between inverter and CU +the value of resistance from substation to CU

 

[trev]

I'm not sure what you're trying to measure and why mate, could you expand a little please?

 

[GaryM]

R1+R2 is the impedance value of the line and CPC conductors combined of the AC cable between the consumer unit and inverter.

Zs is the impedance value of the distributors external line and return earth path (Ze) combined with R1+R2.

How and where was this 1 ohm measurement taken?

 

[OGNIB]

Hi, my inverter keeps triping out on sunny days,the ohm meter reads 1.2 at the inverter end of the AC cable,my installer say the value is only .8 though as he takes the grid value .4 off the 1.2 ,is this correct

 

[OGNIB]

The less than 1 ohm measurement Is what SMA tell me the inverter will work at

 

[GaryM]

Do you know the size of cable? It should be noted on your Electrical Installation Certificate. Also, what's the approximate length?

 

[GaryM]

Hi, my inverter keeps triping out on sunny days,the ohm meter reads 1.2 at the inverter end of the AC cable,my installer say the value is only .8 though as he takes the grid value .4 off the 1.2 ,is this correct

Those figures would suggest:

Ze = 0.4ohms
R1+R2 = 0.8ohms
Zs = 1.2ohms

However, you don't calculate Ze by taking the R1+R2 value away from the Zs value. Ze needs to be measured with the installation isolated and all parallel earths removed. You can take this figure and add R1+R2 to get a calculated Zs figure though in reality the actual Zs figure would be lower due to parallel earths like the bonding conductors.

 

[screwdriver]

Do you know the size of cable? It should be noted on your Electrical Installation Certificate. Also, what's the approximate length?

mmm sounds like another long run of 2.5!!!!

 

[GaryM]

mmm sounds like another long run of 2.5!!!!

Yeah, around 41 metres of 2.5/1.5 t&e!

If it is 2.5 t&e and the length is around 41 metres then the same route using 6mm t&e would give an R1+R2 value of 0.43ohms and a Zs of 0.83ohms at the inverter assuming the quoted Ze figure of 0.4ohms is correct.

A lot of this is assumption though until the OP confirms cable size and length.

 

[OGNIB]

Hi, sorry not got back, internet down, I have a NAPIT ELECTRICAL CERTIFICATE ,is this the cable size,protective bonding conductot 10 mm ,but if that is the cable it is wrong ,it is 6mm 4core twined up so ineffect I been told 12mm.As for the lengh, no sign of it on the certificate but I have measured it ,its 150meters long

 

[OGNIB]

Please tell me if I am correct,R1+R2= resistance value of AC cable from CU to inverter ,Zs = resistance value at the inverter, Ze = resistance value of the grid.

 

[GaryM]

it is 6mm 4core twined up so ineffect I been told 12mm.As for the lengh, no sign of it on the certificate but I have measured it ,its 150meters long

That'll give an approximate R1+R2 value of 0.462ohms, add Ze to that of 0.4ohms which you gave and you have a Zs of 0.862ohms.

 

[OGNIB]

So does the inverter work on the Zs value

 

[OGNIB]

Given all the cable information and the resistance values given, do you think the inverter triping out is a cable problem

 

[SolarCity]

What size system is it?

For comparison, we have used 25mm cable for a 100m run on a 4kWp system. Some may say this is overkill - until they end up replacing a 150m cable.

 

[OGNIB]

4kwp system , cable is 12mm, 150meters long

 

[SolarCity]

The cable is undersized in my opinion.