Disconnection Times

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For a TN system a distribution circuit may have a disconnection time up to 5s, also for circuits not covered by 411.3.2.2 i.e. final circuits greater than 32A.

411.3.2.2 covers final circuits not exceeding 32A
 
OK, so for TN system circuits no greater than 32A, 0.4 s is the disconnection time, for TT, 0.2 s.

For TN >32A or distribution circuits, it's 5 s.

For TT, 1 s.


Question 1.

If TT disconnection times are 0.2 s, how does that reconcile with RCD's disconnecting at anything up to 0.3 s?

Question 2.

Is there a strict definition of a distribution circuit?
 
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Apologies, 123, I can't see for looking. The def is in definitions, strangely... :oops:
 
securespark said:
Question 1.

If TT disconnection times are 0.2 s, how does that reconcile with RCD's disconnecting at anything up to 0.3 s?
Click to expand...
where are you getting your 300ms from?
it's 200ms max isn't it?
unless you use time delay which you're not allowed to do for circuits under 32A ( so regular rcd's for sockets.. ). the main RCD can be time delayed to discriminate with the smaller ones..
 
No, older BS types were 200mS but newer BS EN types are 300mS.

However with a residual current of 5 I delta n, i.e. 150mA for 30mA device it must disconnect within 40mS. Your disconnection times will be met.
 
The key to the time delay RCD time issues is buried in the EN (61008 RCCBs or 61009 RCBOs) for RCDs, they must disconnect within 500mS at the rated trip current, 200mS for 2 x rated trip current, 150mS for 5x trip current.
The table in 411.3.3.2 requires the RCD to trip when the leakage current is significantly higher than the rated trip current of the device, it was originally printed as 2x but the 2x figure was later deleted by corrigendum no 1, probably as nobody had a tester!
If you still read it as 2x then a time delayed RCD will disconnect within 200mS as required by 411.3.3.2.
 
HI folks, recently I had to install an Earth rod at a property. The Ze reading was 13.4Ohms.

So naturally when carrying out Zs readings I expected them to be higher and not meet current maximum values.
This is the only time I have come across a TT at work and am struggling to see how to reconcile these values, the property is going to be fully refurbed soon

I remember Securespark? on about compiling a TT readings guide quite a while ago
 
13.4Ω seems perfectly OK for a TT Ze - well under the threshold of what is viewed as potentially unreliable.

Don't see what the problem is..... :confused:
 
oharaf said:
HI folks, recently I had to install an Earth rod at a property. The Ze reading was 13.4Ohms.

So naturally when carrying out Zs readings I expected them to be higher and not meet current maximum values.
This is the only time I have come across a TT at work and am struggling to see how to reconcile these values, the property is going to be fully refurbed soon

I remember Securespark? on about compiling a TT readings guide quite a while ago
Click to expand...

Remember though that Zs tests have to be carried out with all MPBs in place so will more than likely lower the readings to below the stand alone Ze value.
 
oharaf said:
So naturally when carrying out Zs readings I expected them to be higher and not meet current maximum values.
This is the only time I have come across a TT at work and am struggling to see how to reconcile these values, the property is going to be fully refurbed soon
Click to expand...
If you can't meet the max Zs value for an MCB/fuse then you need to use RCD protection for all connected circuits and the respective RCD max Zs figures, with a Ze of 13.4 ohms these are easy peasy to meet, for example if you have a max touch voltage of 50v and are using a 30mA RCD then the max theoretical Zs for the circuit is 50/0.03 = 1666 ohms, although in reality if you measured anything like that you'd be looking for a problem!
 
D

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