Zs on tt

[james king]

hi all am i right in saying on tt systems the Zs reading will be a lot higher than the max? and is this exeptable?

 

[IQ Electrical]

hi all am i right in saying on tt systems the Zs reading will be a lot higher than the max? and is this exeptable?

It will depend on what system you have in place to enable disconnection times to be met (think RCD's).

 

[tony mc]

ZE can be higher up to 200 ohms but you want it as low as possible, ZS will hopefully come down with parralell paths bonding etc.

TT up to 200 OHMS
TNS up to 0.8
TNCS up to 0.35 OHMS

 

[nickblake]

Zs or Ze on a TT system the Ze should be less than 200ohms , the maximum Zs for correct operation is 1667 for 30ma RCD's and details found on table 41.5 page 50 of the regs , if you read reg 411.5 of the big red book

 

[the pict]

whats the ZE has to be 200ohms at least I did one today turned out there is no rod at the end of the main earth conductors pikeys probably had it away,
Pict

 

[Sintra]

The Zs on a TT will be higher than on a TN system but it can not exceed the max permitted.

Max permissible Zs on a 230V circuit with a 30mA RCD as additional protection is 1667 ohms.


Sent from my iPhone using Tapatalk.

 

[sjm]

As Zs=Ze+R1+R2 then yes Zs on a TT system will generally be much higher than a TNCS system.
The maximum values are given for TNCS systems and therefore are not applicable to TT systems. The maximum value given in BS7671 is, I believe, 1667 ohms.

 

[james king]

say the Zs on a ring main is 3.45ohms or higher on a TT are u saying 1667ohms is the max?

 

[james king]

and has RCD protection

 

[IQ Electrical]

say the Zs on a ring main is 3.45ohms or higher on a TT are u saying 1667ohms is the max?

Yes, for a 30mA RCD/RCBO protected circuit.

Are you working on such a system or are these study questions?

 

[james king]

im only wondering.. looking through Qs and though of that exapmle thts all

 

[sjm]

As the other guys have said you must have RCD protection for a TT system.
3.45 ohms Zs is very good for a TT system, what's the Ze?
Yes 1667 ohms is the max value for your certificate IMO.

 

[IQ Electrical]

As Zs=Ze+R1+R2 then yes Zs on a TT system will generally be much higher than a TNCS system.
The maximum values are given for TNCS systems and therefore are not applicable to TT systems. The maximum value given in BS7671 is, I believe, 1667 ohms.

The tabulated values in table 41.3 are for TN systems (TN-S AND TN-C-S)

 

[james king]

does this 1667ohms mean the max for all mcb sizes? o and what does IMO mean always wondered

 

[IQ Electrical]

does this 1667ohms mean the max for all mcb sizes? o and what does IMO mean always wondered

For any device with or covered by 30mA rcd protection.

IMO means In My Opinion

 

[Sparky Ninja]

If people are having trouble understanding this magic '1667 ohms' number.. it is obtained from:

50V / 30mA = 50 / 0.03 = 1666.6666666666666666666666667, rounded up to 1667 ohms.

 

[jaresquire]

I think you missed a few 6's there, Widdler

 

[sparx24]

was about to say as widdler max. touch voltage 50 so 50/0.03mA = 1666 recuring rounded to 1667, but OSG says anything over 200 ohms should be considered unreliable due to soil drying out ect.
TT probs with B.Gas 'engineers' having plug in Go/noGo testers for supply to boilers having fail setting of a couple of ohms, trouble is they are always taught on TNS systems and don't understand TT.

 

[IQ Electrical]

was about to say as widdler max. touch voltage 50 so 50/0.03mA = 1666 recuring rounded to 1667, but OSG says anything over 200 ohms should be considered unreliable due to soil drying out ect.
TT probs with B.Gas 'engineers' having plug in Go/noGo testers for supply to boilers having fail setting of a couple of ohms, trouble is they are always taught on TNS systems and don't understand TT.

No, the On-Site Guide is referring to the RA value (the Ze if you like) NOT Zs values!