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Topic Title: Periodic condition report
Topic Summary: Zs and a BS88 355Amp fuse
Created On: 20 February 2014 10:56 PM
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 20 February 2014 10:56 PM
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Joined: 04 August 2011

An installation installed during the IEE reg's 14th edition, (yes, I am ancient) and in overall good condition, easily complies with the earth loop impedance in force at the time. I checked and calculated that from my copy.
Using the IET wiring reg's 17th edition amended 2011.Calculating the disconnection time as 5seconds, the Zs of the circuit is too high by x4, requiring a C2 on the form.
I understand that times change and that standards alter, and I don't argue with the present requirements for a new installation, but for the life of me I fail to see how this situation can be considered a potential electrical hazard. I compare my 1971 built motorcycle does not comply to modern standards, but it passes the MOT.
 21 February 2014 05:03 AM
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"Zs to high by 4x" what calculation did you use? The 14th or 17th edition give no values for fuses of 355A?
I personally would have another look at your calculation and compare it with device manufactures max values and see where you go from there

MrP day two
 21 February 2014 10:55 AM
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Avatar for OMS.

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Joined: 23 March 2004

I very much doubt it "easily" complies at 240V and fails at 230V given that the difference in tabulated values is basically 230/240 x Zs

If it's a 315A fuse then a max Zs around 0.12 Ohm in the old money would "translate" to about 0.11 Ohm in the new money - so I suspect given any sensible approach to temperature under fault and possible voltage depression under fault, then it was always very borderline, if not a fail

Personally, I wouldn't have any problem for an EICR in re adjusting max Zs at 230V back to max Zs at 240V for a compliance check if required. Note what you've done of course, but at worst it would attract a Code 3



Failure is always an option
 21 February 2014 02:36 PM
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John Peckham

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I to am not that ancient but I have on my book shelf the 14th Edition and earlier editions back to the 9th edition 1927.

I selected one of my copies of the 14th and found that the requirement therein was for the earth fault current to be 3 times the rated current of the fuse. So that would be 3 x 315A = 945A. Using a supply voltage of 240V that would mean a Zs of 0.25 ohms.

You say the Zs was to high by a factor of 4 against the requirements of the 17th. So that would be 0.11 x 4 = 0.44 ohms. So even based on the 14th Edition the installation was non-compliant then and now. I have not assumed any rise in conductor temperature so things could be worse.

If you do have a measured Zs of 0.44 ohms that would mean a fault current of 240/0.44 = 545A. That would mean a disconnection time of about 2 hours for a 315A fuse.

Aside any short circuit protection for the installed cable a 2 hour disconnection time is not a very safe condition!

What was the measured Zs?

So a Code 2 for me.

John Peckham
 25 February 2014 09:11 PM
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Thanks for the response. I don't dispute the code, greater minds than I write the regulations, but...
Using the characteristic curves for the bs88 355amp (available at Busman fuses) fuse I calculated the approximate Zs = 0.06 ? for a 5 second disconnection time. I will not bother the rule of thumb for the sake of clarity.
With reference to the 14th edition of the IEE wiring regulations, look at D22, it's x 2.4 the rating of the cartridge fuse = 852A = 0.27 ?, @ 230v (rounded up to two decimal points)
At that time the nominal voltage was 240V. =0.28?
The recorded reading of the circuit in question was actually 0.24?
 25 February 2014 10:16 PM
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John Peckham

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Ok I thought 355A was an error so looked at a 315A fuse. I have now plotted the curve for 355A fuse on my Amtech protect.

A Zs reading of 0.24 ohms is giving me a disconnection time of 123s at 230V, and 925s at 240V. So well above the required 5s disconnection time, so not a lot of shock protection.

So I am still going for a Code C2 on an EICR.

PS what have the last 30 years of PIRs said about this Zs?

John Peckham

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