Maximum permitted Zs

[Des 56]

My take on mcb operation on a TT system

We test the prospective short circuit current at the board,that is the highest value
A circuit with a dead short will operate the mcb
Overload will occur with a poor circuit neutral,some of the current converting to heat at the connection,the mcb will still operate until the connection becomes too bad

A poor neutral connection may cause some overheating,but we tick a box to say the connections are secure,so that should cover the poor neutral bit,

A poor neutral connection may not impact on the current flow of a dead short
A poor neutral may stop the flow of current so no need for the mcb to see it

This seems to me to be the reason why Rn is not asked to be measured
It still seems like a sensible test to do though

What is others opinions on TT mcb operation ?

 

[SimpleSimon]

An MCB on TT system acts just like it would on a TN I was lead to understand.
If this isnt the case why does BS7671 only have time curve tables for MCB operation that aren't system specific?

MCB. (In my own words without the BRB in front of me.)
A device for fault protection in the event of a short L-N. MCB's also provide protection against overload of a circuit.
The tables in BS7671 show the 3 types of MCB (B/C/D) and the Ia that provides disconnection time in the allowed time (0.4-5s) for overload (for B type it's about x5 the mcb current rating?) regardless of system...

I've read this thread but I've lost the gist of what it started out as.

 

[pushrod]

An MCB on TT system acts just like it would on a TN I was lead to understand.
If this isnt the case why does BS7671 only have time curve tables for MCB operation that aren't system specific?

MCB. (In my own words without the BRB in front of me.)
A device for fault protection in the event of a short L-N. MCB's also provide protection against overload of a circuit.
The tables in BS7671 show the 3 types of MCB (B/C/D) and the Ia that provides disconnection time in the allowed time (0.4-5s) for overload (for B type it's about x5 the mcb current rating?) regardless of system...

I've read this thread but I've lost the gist of what it started out as.

An MCB will try and act the same way with both a TT and a TN. With an earth fault on a TN such a large current should flow (because of the low Zs) that the MCB should trip, (if there is an rcd that will trip as well). However on a TT system because the max Zs can be up to 1667 ohms it is unlikely that an earth fault will take out the MCB, just the rcd will trip.

The discussion has moved on to ask whether people check Rn, even though it is not asked for, to check that a dead short will activate the MCB in the event of a short. The Zs value is only really relevant for earth faults.

With a dead short it is the impedance on R1 and Rn rather than R1 and R2 that will determine the fault current.

 

[SimpleSimon]

Agreed.
I don't check for Rn, it's never crossed my mind. But what values do we check Rn against to ensure that it's impedance is low enough in a dead short?
Or are we just checking for continuity of Rn?

 

[pushrod]

Agreed.
I don't check for Rn, it's never crossed my mind. But what values do we check Rn against to ensure that it's impedance is low enough in a dead short?

If R1+Rn is equally as low as R1+R2 (in theory it should be slightly lower as the cpc will probs have a smaller csa) then you can check it against max Zs values as in appendix 2 of the OSG.

 

[SimpleSimon]

So real life scenario.

Today's installation was about a new supply fed to a new CU but earth electrode on a TT remained the same but was moved across the house (bout 10m).

Both old and new supplies where protected by an RCD.

Old Ze= 157.
New Ze= 214.

So according to BS7671 1667 is the Max.

Calculate Ra x In < 50v.
(0.03 x 214)= 6.42v.

So everything works fine yes? Although I know the Ra is over 200 ohms slightly is should be ok? Soil resistivety I know, maybe I whacked it in too hard

 

[kevinjstanley]

Hi people

the max zs of a rcd protected circuit on a 32 amp breaker is as you have said 1.44 or 1.15, this impediance can be improved ? what i struggle with is how to determine what is causing a high reading.
is it that there are items plugged in or bad terminations?

 

[malcolmsanford]

The maximum Zs for an RCD protected circuit is 1667 the 1.44 or the 1.15 is redundant now as you would only be using the MCB for overload protection.

But for argument sake if your getting a high Zs of say 1.6 on an MCB protected circuit then it would most likely be a poor connection. It could be poor design ie cable length, wrong size cable. If you are taking a reading live from a BS 1363 socket it may also be dirty pins, which is why if testing on an older installation on a PIR and you get an high Zs put the plug in and out a few times to clean them. Also some cheaper brands of sockets can also give you an higher Zs due to quality.

 

[kmcgold]

i always use zs for the mcb /rcbo @ 80%(protective device for that circuit)
rcd in a TT system - fault protection
rcd in TN system - additional proctection
i always measure the zs with a meter (i know you can just add ze+r1+r2)
maximum zs for mcb to bs 60898 and a rcbo to bs 61009 are the same.

 

[brizospark]

the rcd is only in the event of an earth fault - in the event of a L-N short the rcd would not trip.
table 41.5 is only for earth faults. The low Zs ensures mcb operation in the event of a short.

That aint true mate, an RCD constantly monitors the current on both phase and neutral conductors therefore if there is a fault with either the RCD will trip , providing the fault is more than 30mA

 

[pushrod]

That aint true mate, an RCD constantly monitors the current on both phase and neutral conductors therefore if there is a fault with either the RCD will trip , providing the fault is more than 30mA

An RCD works on the current balance between L and N and the subsequent magnetic fields being balanced. A residual current difference of 30mA+ will generate a sufficient magnetic force to cause a trip. In the event of an earth fault the balance between the L current magnetic field is no longer balanced by the N current's mag field (some of the current is now travelling direct to earth) and so the DP switch is tripped. If it is a short (between L and N) then a larger current will flow in L due to the lower resistance but it will return via the N and so the balance is maintained and the device does not trip. However the larger current flow should trip the mcb.

If what you said was true there would be no point in having mcbs as well as rcds on the same circuit!!! An rcd is an earth fault device are you thinking of an RCBO?