Main Earth CSA TT Encorporated in a cable

[tudge56]

Hello All,
Tying my brain in knots trying to work out if this earth is ok:
TT system with 100mA RCD on tails
Tails then go into an isolator with 60A BS1361 fuse which supplies nearby consumer unit and a remote consumer unit in next room.
Remote consumer unit is supplied via twin and earth 10mm (so 4mm CPC - am I right?)
So applying the adiabatic equation to this, my problem is:
Earth rod resistance is 4.05ohm and PFC (to earth) 56A....therefore my 60A fuse is never going to blow so I can't get a 'time' number for the equation.
Obviously the 100mA RCD would trip in the event of an earth fault, but it isn't an RCBO so isn't listed in appendix 3 (time current characteristics) so I'm not sure what to do!

Any help much appreciated

 

[Pete999]

Hello All,
Tying my brain in knots trying to work out if this earth is ok:
TT system with 100mA RCD on tails
Tails then go into an isolator with 60A BS1361 fuse which supplies nearby consumer unit and a remote consumer unit in next room.
Remote consumer unit is supplied via twin and earth 10mm (so 4mm CPC - am I right?)
So applying the adiabatic equation to this, my problem is:
Earth rod resistance is 4.05ohm and PFC (to earth) 56A....therefore my 60A fuse is never going to blow so I can't get a 'time' number for the equation.
Obviously the 100mA RCD would trip in the event of an earth fault, but it isn't an RCBO so isn't listed in appendix 3 (time current characteristics) so I'm not sure what to do!

Any help much appreciated

Did you diss the Main Earth when carrying out your tests?

 

[Wilko]

The Earth fault is covered by RCD and the fuse is for short circuit faults

 

[Ian1981]

Thats quite a good reading for a TT (4 ohms)
Did you disconnect any possible parallel paths like gas and water if they have protective bonding to them?
Did you carry a out an rcd test on the 100mA rcd?
Not sure what the Adiabatic equation has got to do with anything

 

[westward10]

The Earth fault is covered by RCD and the fuse is for short circuit faults

And overcurrent

 

[bigspark17]

What ref method is that 10mm t&e? If be more concerned with ccc being on a 60A fuse.

 

[Deleted member 26818]

The minimum CSA allowed for an Earthing conductor is 2.5mm2.
Selection is determined by means of the adiabatic equation.
Use a value of 115 for k, 1s for t and whatever you measure the PEFC for I.

 

[Ian1981]

If your fault current is only 56 amps then t can’t be 1 second.
T is found using the curves from appendix 3 depending on the fault current and the protective device
The rcd is there to give you the fault protection as you can rarely achieve ads on a TT without an rcd.
As the op says 56 amps will never blow a 60 amp fuse
2.5mm can be used on a TT if the cable is protected from corrosion and mechanically protected and not directly buried

 

[westward10]

Do we know the earthing conductor size?

 

[Richard Burns]

You have a 100mA RCD to disconnect the circuit in the case of an earth fault. A 100mA RCD must disconnect in 300ms or less at its rated current (100mA).
Since your measured earth fault current is 56A the RCD will (if working correctly) disconnect within 300ms (probably much less).
If you were to use the adiabatic equation with these values the csa would be tiny (0.27mm²), therefore the cpc is suitable for protection against earth fault current.

 

[Deleted member 26818]

1s is the maximum time allowed for disconnection of this type of circuit.
Do we know if the 100mA RCD is time delayed?

 

[Ian1981]

It’s the max time allowed yes but it’s not the figure you would use in the equation.
As I said t would be determined on the fault current
As an example a fault current of 600amps would take 0.4 seconds to disconnect a bs 1361 fuse so if your fault current was 600 amps then t in the equation would be 0.4 seconds regardless of what the max disconnection time is for the circuit

 

[Ian1981]

You have a 100mA RCD to disconnect the circuit in the case of an earth fault. A 100mA RCD must disconnect in 300ms or less at its rated current (100mA).
Since your measured earth fault current is 56A the RCD will (if working correctly) disconnect within 300ms (probably much less).
If you were to use the adiabatic equation with these values the csa would be tiny (0.27mm²), therefore the cpc is suitable for protection against earth fault current.

It has to disconnect in 300ms however if it was protecting a circuit upto 32 amps on a tt then it must disconnect in 200ms or less for compliance

 

[Richard Burns]

It has to disconnect in 300ms however if it was protecting a circuit upto 32 amps on a tt then it must disconnect in 200ms or less for compliance if non time delayed

The Op example is a distribution circuit protected at 63A.
You would be correct if the circuit was a final circuit and 32A or less.
However most RCDs will trip in far less than the limiting time and the actual trip time recorded, even if only at 100mA, could be used as a worst case scenario where the fault was not of negligible impedance. Since the trip time at a higher current will not be less than the recorded time.

 

[Strima]

What is the PSCC?