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# Max Zs on a 30ma RCD

Discuss Max Zs on a 30ma RCD in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

C

#### claire_r

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Hey all, I seem to have it in my head that the max zs, regardless of what breaker you do the test from, on a 30ma rcd is 1667 ohms. But I can only find a reference to this when it comes to tt systems. Does this still apply on tns, tncs systems and if so where is it in the regs? Thanks all in advance, Claire

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A

#### acat

Hi Claire

you are looking at page 50 of regs have a look at the top of page 49 I think that covers the rest

Chris

#### Mark.W

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Look at table 41.5 in the regs

30mA max Zs is 1667 Ohms if the voltage is greater than 50V but less than or equal to 120V
30mA max Zs is 1667 Ohms if the voltage is greater than 120V but less than or equal to 230V
30mA max Zs is 1533 Ohms if the voltage is greater than 230V but less than or equal to 400V
30mA max Zs is 1667 Ohms if the voltage is greater than 400V

Hope this helps

Mark

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S

#### Spudnik

The maximum Zs for fuses/mcb's etc is listed in the tables in 7671.

There is NO maximum Zs for a 30mA RCD.

Therefore for TN systems, these values should not be exceeded, to achieve the required disconnection times.

In the case of TT systems, these values can rarely be achieved and so an RCD should be used as additional protection.

The use of 30mA RCD's are allowed in TN systems if the Zs values cannot be met.

This however, is NOT a get out clause and reasonable steps must have been taken to try and meet the values given.

G

#### Guest123

This however, is NOT a get out clause and reasonable steps must have been taken to try and meet the values given.
Took the words right out of my mouth.

It worries me, the amount of people I hear talking in wholesalers and the like about "dont worry about the fault loop, It's got an RCD"

As you say RCD's are for additional protection and the requirement to meet the tabulated values for a TN system should not be disregarded.

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S

#### spark1

No Claire, on TNCS & TNS systems the RCD is usually installed as "Additional Protection" to the failure of Basic Protection. Overcurrent and Fault protection is required ,and the Fault Protection element is provided bythe C.B ,--Fuse--,or C.B part of an RCBO,Therefore Tables 41.2,41.3 or 41.4 need to be complied with.
Table 41.5 is usually utilized on TT systems for Earth Fault protection where Ze values are to high to allow adequate Zs values to be accomplished to meet disconnection times

spark1

Apologies to Jason & Lenny for basically repeating what you have just posted .

spark1

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#### Mark.W

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Thanks Jason.S, I misunderstood the question. My eyes somehow skipped over the part where Claire asked if it applied to a TN system.

Regards
Mark

C

#### claire_r

Thanks all, been a great help. Think I had to remember this magic 1667 number for my 2391 so ever since then i think its the answer to everything hehe. Soooooo with 1.11 ohms being the max reading for a 32a mcb, how much over that reading would you guys let off. Or is that the be all and end all of readings??

S

#### spark1

Is your quoted figure of 1.11 your adjusted 8o% value, I dont regonize that figure at all.
The max allowable Zs is 1.44 ohms for a bs 60898 32A mcb ,with an acceptable practal thermal adjusted figure being 1.15 ohms

spark1

#### scotsparky

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Arms
In TT systems remeber fault voltage everyone!!!

Ra x In < 50V

In being the rated value of the rcd 30ma.
Ra is end of CPC and earth. If you dont know the Ra use Zs.

This applies when using Rcds,Rcbos for earth fault protection.

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