Maz zs

[Catweasel]

When completing any cert if the device was say a bs 61009 6 amp type b 30 mA would the max zs written on the cert be 7.67 ohms for a B6 or 1667 ohms thanks

 

[telectrix]

Some say, if RCD protected, 1667.otherwise, 7.67

 

[jpmiff]

correct me if im confused

iisnt the 1667 for TT systems?

 

[Deleted member 9648]

The maximum allowable Zs where a circuit is protected by a 30ma RCD whether the system is TT or TN is 1667 ohms. On a TN system Zs readings would normally expected to be within the range of the overcurrent device,and an excessively high zs would be unacceptable.But the fact remains that under bs 7671 the max allowable Zs is 1667 ohms for a 30ma protected circuit and so that value can be used on the cert.

 

[telectrix]

The maximum allowable Zs where a circuit is protected by a 30ma RCD whether the system is TT or TN is 1667 ohms. On a TN system Zs readings would normally expected to be within the range of the overcurrent device,and an excessively high zs would be unacceptable.But the fact remains that under bs 7671 the max allowable Zs is 1667 ohms for a 30ma protected circuit and so that value can be used on the cert.

agreed, but what value do you yourslf enter on the cert.? must admit, 1667 saves looking up figures in osg!

 

[IQ Electrical]

I always enter 1667, BS7671 does not make a distinction between earthing systems when arriving at this value.

As long as the circuit has been checked for continuity and rcd function tests have been carried out, then requirements for automatic disconnection are satisfied.

 

[Deleted member 9648]

agreed, but what value do you yourslf enter on the cert.? must admit, 1667 saves looking up figures in osg!

I enter 1667 cos as you say,it's easier than looking up the value for the mcb....we do a lot of small jobs which means a lot of certs and anything which makes it less of a chore gets my vote.
That said on our recent NIC annual inspection the guy picked this up on my certs and said I should enter the value for the MCB,not the 1667...I wasnt present to query this but after re-checking the regulations I am confident he is incorrect and will continue to use the 1667 value as a max on the cert. Either value can be correctly used,for me I can remember the 1667 which makes life easier.

 

[Graeme Harrold]

I would always insert the max permissable value fro the MCB type regardless of the RCD. Its good practice to ensure you install within the max zs values. I was called out to a boiler fault recently (ended up as an internal fault) that knocked out the RCD when it fired up. I plugged into a nearby socket to do a ramp test on the RCD, it failed to trip. I then did the full range of RCD time tests X1/2 OK, x1 OK but x5 failed to trip within time/voltage limits. Next I conducted a Zs on the socket which came back at 548 ohms!!!! Only once I had reconnected the earth supply did everything come back to realistic values.

An RCD is ADDITIONAL protection not a get out clause for either not knowing the Zs limits or not having circuits that fall within the max Zs limits.

 

[IQ Electrical]

I would always insert the max permissable value fro the MCB type regardless of the RCD. Its good practice to ensure you install within the max zs values. I was called out to a boiler fault recently (ended up as an internal fault) that knocked out the RCD when it fired up. I plugged into a nearby socket to do a ramp test on the RCD, it failed to trip. I then did the full range of RCD time tests X1/2 OK, x1 OK but x5 failed to trip within time/voltage limits. Next I conducted a Zs on the socket which came back at 548M ohms!!!! Only once I had reconnected the earth supply did everything come back to realistic values.

An RCD is ADDITIONAL protection not a get out clause for either not knowing the Zs limits or not having circuits that fall within the max Zs limits.

I agree it's good practice but as BS7671 permits it, then we can use the 1667 value on all certification and that is the answer to the original question!

 

[telectrix]

this ties in with my post " unacceptable Zs/Ze values" . wasthe electrician correct in refusing to connect a new socket when his Zs was 51ohms due to a Ze reading at the house ( cut-out is 80m away) was 480hms. Ze at cut-out is 0.23.

 

[Guest123]

Old habbits die hard for me. On TN systems pre 17th ed, lights, immersion etc were always on the unprotected side of a split load CU so the the MCB values were put in on those systems for both sides of the CU. TT would have the 1667ohms figure for 30mA protected circuits and 500ohms for 100mA etc.

 

[Graeme Harrold]

Even though 1667ohms is permitted I have found instances where the RCD will not trip (especially on 5X) within the prescribed time/voltage rise as per my previous post. I still put the actual max Zs as per the CB rating and if the measured Zs is higher I will investigate. If all checks out and its still high, then Ill add it as an observation on the cert.

 

[Deleted member 9648]

I would always insert the max permissable value fro the MCB type regardless of the RCD. Its good practice to ensure you install within the max zs values. I was called out to a boiler fault recently (ended up as an internal fault) that knocked out the RCD when it fired up. I plugged into a nearby socket to do a ramp test on the RCD, it failed to trip. I then did the full range of RCD time tests X1/2 OK, x1 OK but x5 failed to trip within time/voltage limits. Next I conducted a Zs on the socket which came back at 548 ohms!!!! Only once I had reconnected the earth supply did everything come back to realistic values.

An RCD is ADDITIONAL protection not a get out clause for either not knowing the Zs limits or not having circuits that fall within the max Zs limits.

Absolutely,nobody here is saying that a very high Zs is acceptable.....any issues would or should be picked up prior the the Zs test when Ze and R1R2 are tested....the question is which value should be used as the max on the cert, MCB or RCD? the answer is both,whichever takes your preference.

 

[Deleted member 9648]

Even though 1667ohms is permitted I have found instances where the RCD will not trip (especially on 5X) within the prescribed time/voltage rise as per my previous post. I still put the actual max Zs as per the CB rating and if the measured Zs is higher I will investigate. If all checks out and its still high, then Ill add it as an observation on the cert.

So what do you put when dealing with a TT system?.....you have a measured Zs of 195 ohms..the circuit is protected by a 32a type B and a 30ma rcd,are you going to write in a max allowable Zs of 1.15 ohms and then record a Zs of 195 ohms?.....presumably not,you will record the max Zs as 1667 ohms......so why not with a TN system?

 

[SimpleSimon]

And so the argument continues