TT Max ZS loop reading for circuits over 32A

[Welchyboy]

HI all anyone know the above, i put 1667 ohms in the box normally for all TT circuits but noticed the other day it is only for circuits upto 32A

Anyone enlighten me, i need it for a 50A and 40A circuit??

thanks

 

[spark 68]

Hi,
The 1667 ohms refers to the max ZS for a 30mA RCD to meet the disconnection time, but over 200 ohms may be unstable.

BTW, I use the BS7671 certs from the IET site, and I cannot see where to put the max ZS ie. 1667 ohms for TT on this cert. I usually just put my measured/calculated results on the schedule of test results for each particular circuit, and obviously the measured ZE/RA on the relevent sheet.

 

[topquark]

Regulation 411.3.2.4 applies with TT type earthing systems for distribution circuits (and circuits over 32A). Disconnection time is not allowed to exceed 1 second for the protective device. I can't imagine why you would allow a higher max Zs than 1667Ohms, as it's purpose is to ensure disconnection before the PD reaches 50V, unless I'm missing something.

 

[Welchyboy]

no that what i thought but i just thought there may be another standard value for above 32A circuits

 

[topquark]

Just grabbed the BRB (BGB on order) and 411.5.4 would also apply without an RCD ... ie Zs x Ia <= U0 must be satisfied ... which would give:

for 40A) Zs x 40A <= 230 which would give Zs as max 5.75 Ohms
for 50A) Zs x 50A <= 230 which would give Zs as max 4.6 Ohms

You must also meet the disconnection time of 1S.

 

[GLENNSPARK]

TT systems: 411.5.3 (p50)
Where an RCD is used for earth fault protection, the following conditions shall be fulfilled:
(i) the dissconection time shall be that required by regulation 411.3.2.2 or 411.3.2.4 and
(ii) Ra x I delta n< 50V
As topquark...

 

[topquark]

I think the second post confused the issue a bit, the OP made no mention of RCDs

 

[spark 68]

With TT circuits, all circuits must be protected by an RCD as TQ said, regardless of the size of the over current protection device.

The max ZS refers to the trip current rating of the RCD, again as TQ said to prevent the PD rising above 50V (touch voltage).

30mA = 1667 ohms
100mA = 500 ohms
300mA = 167 ohms
500mA = 100 ohms

these are the absolute max values, and in practice we should aim to be as low as possible.

Ps. in some special locations the touch voltage may be limited to <= 25V, and these require an even lower max ZS and/or RA

Sorry to confuse, the joys of forum posting

 

[GLENNSPARK]

I think the second post confused the issue a bit, the OP made no mention of RCDs

No probs mate...Sorry about that TQ Will have to read the post a bit clearer next time lol......:smile5:

 

[topquark]

With TT circuits, all circuits must be protected by an RCD as TQ said, regardless of the size of the over current protection device.

The max ZS refers to the trip current rating of the RCD, again as TQ said to prevent the PD rising above 50V (touch voltage).

30mA = 1667 ohms
100mA = 500 ohms
300mA = 167 ohms
500mA = 100 ohms

these are the absolute max values, and in practice we should aim to be as low as possible.

Did I say that?

 

[topquark]

To be fair there is no rule that says that all circuits have to be. It's just difficult to get Ze(Ra) low enough to get the Zs low enough to comply without an RCD. Keep in mind distribution circuits may not want to be RCD protected at source, unless using a slower trip to get some discrimination.

 

[GLENNSPARK]

To be fair there is no rule that says that all circuits have to be. It's just difficult to get Ze(Ra) low enough to get the Zs low enough to comply without an RCD. Keep in mind distribution circuits may not want to be RCD protected at source, unless using a slower trip to get some discrimination.

Well that was the idea behind 100mA time delayed`s wasn`t it?..........

 

[topquark]

Well that was the idea behind 100mA time delayed`s wasn`t it?..........

Yep

Just trying to make the point that if it's TT it doesn't have to have an RCD to comply (although it's unlikely without).

 

[IQ Electrical]

With TT circuits, all circuits must be protected by an RCD as TQ said, regardless of the size of the over current protection device.

The max ZS refers to the trip current rating of the RCD, again as TQ said to prevent the PD rising above 50V (touch voltage).

30mA = 1667 ohms
100mA = 500 ohms
300mA = 167 ohms
500mA = 100 ohms

these are the absolute max values, and in practice we should aim to be as low as possible.

Ps. in some special locations the touch voltage may be limited to <= 25V, and these require an even lower max ZS and/or RA
Sorry to confuse, the joys of forum posting

I don't think you'll find a 25V touch voltage stipulated anywhere in BS7671:2008

 

[GLENNSPARK]

I don't think you'll find a 25V touch voltage stipulated anywhere in BS7671:2008

nope....Max 50V......