Max Zs for TT inatallation

[brizospark]

Are the values for max Zs values permitted for a type of circuit breaker the same regardless of whether a TT or TN system?

The reason i'm asking is because the disconnection for a TT ring main is 0.2 opposed to that of a TN 0.4

 

[telectrix]

on TT the max. is 1667ohms. RCD essential to achieve 0.2 sec.

 

[IQ Electrical]

Note that the 1667 Ohm value assumes an RCD with a rated residual operating current of 30mA.

Where an RCD is used, the product of the rated residual operating current, in amps and the sum of the earth electrode and earth conductor resistance (RA), in Ohms must not exceed 50 Volts.

 

[w crane]

Sorry having a bad day. i understand the concept behind touch contact of 50v and the above 1666 ohms. but when physically testing with 17th ed dilog. testing TT system 30mA RCD at 1x and 5X what reading would you see on your meter? or would you just test the mechanical aspect of the RCD ( test button)

 

[IQ Electrical]

Sorry having a bad day. i understand the concept behind touch contact of 50v and the above 1666 ohms. but when physically testing with 17th ed dilog. testing TT system 30mA RCD at 1x and 5X what reading would you see on your meter? or would you just test the mechanical aspect of the RCD ( test button)

The readings you are looking for assuming an rcd to BS61009 or BS61008 are:

X1 trip times of less than 300mS

X5 trip times of less than 40mS

If you have a device to BS4293 then X1 should be less than 200mS and X5 is as above.

 

[telectrix]

thought x1 on a TT was 0.2 sec

 

[IQ Electrical]

No, maximum TT system disconnection time is 0.2 seconds but this is at a fault of negligible impedance.
It was explained like this:

A 30mA device will trip in 150mS at 2 x IdeltaN (Appendix 3). This meets the criteria for Table 41.5.

If you read the note to Table 41.2 and the Note to 411.4.4 amended by the corrigendum then the assumption is that currents significantly greater than IdN flow - this is derived in part from the limiting Ra values in Table 41.5 which are designed to ensure significantly greater currents than IdN flow.

So if you take 230 and divide it by a thoretical 1667, you get around 130mA which is greater than the 2X IdN from appendix 3 that tells us the device will disconnect in 150mS or less, thereby complying with table 41.5.

This is why some continentals introduced the 2XIdN test.

 

[BigBreakfast]

Hi All,

just going to add that if the installation is wired in accordance with reg 411.3.1.2, and disconnection is achieved by an overcurrent protective device, then the max disconnection times for a circuit not exceeding 32A in a TT system may be treated as a TN system as noted in table 41.1 BRB.

 

[IQ Electrical]

Hi All,

just going to add that if the installation is wired in accordance with reg 411.3.1.2, and disconnection is achieved by an overcurrent protective device, then the max disconnection times for a circuit not exceeding 32A in a TT system may be treated as a TN system as noted in table 41.1 BRB.

That is true but the note you refer to stipulates that disconnection is achieved by an OVERCURRENT PROTECTIVE DEVICE and I don 't think I've ever seen that on a TT system.

 

[telectrix]

That is true but the note you refer to stipulates that disconnection is achieved by an OVERCURRENT PROTECTIVE DEVICE and I don 't think I've ever seen that on a TT system.

some great long fat earth rods in some installations to achieve disconnection times for overcurrent devices methinks

 

[Des 56]

some great long fat earth rods in some installations to achieve disconnection times for overcurrent devices methinks

There were some extra good readings by some members on a past thread
Most must have been next door to a Tnc-s and reading through the bonding possibly, but very low indeed

 

[johnboy6083]

i would still rely on an RCD, as the Ra can fluctuate due to ground conditions. Ie, wet = low readings, prolonged dry spells = higher readings.

would you gents agree?

 

[IQ Electrical]

i would still rely on an RCD, as the Ra can fluctuate due to ground conditions. Ie, wet = low readings, prolonged dry spells = higher readings.

would you gents agree?

I certainly would, not worth the risk.