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Topic Title: Max Zs Values-
Topic Summary: Think I'm Losing it!
Created On: 12 August 2014 04:25 PM
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 12 August 2014 04:25 PM
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Sanft

Posts: 37
Joined: 22 August 2011

Afternoon Guys and Gals-

Just wondered if you could assist with something that my brain cannot appear to wrap itself around! And yet I think it is relatively simple too!

It surrounds Max Zs Values and I'm curious as to why temperature governs the Max Zs of a given circuit protective device, I'm probably best to give an example-

A 30m 2.5mm/1.5mm Circuit is Run to supply a 3kW Water Heater and is supplied via a 20A Type B 60898 MCB- Which we all know the BGB states the Max value of 2.87 Ohms (@ 70 Degrees)

Now that I think about it, the example is in fact somewhat irrelevant to my question, as I'm curious as to why the Max Permitted Zs is lowered if say, the circuit was in a position to be running at 40 Degrees? (1.16 * 1.12 = 1.30 Ohms Approx)

I was always under the impression that with a higher temperature, there is a larger resistance- and quite frankly if 1.30 Ohms was to be the max value, this would still trip the MCB is <0.1 seconds as per the graphs in the BGB (assuming the voltage is 230v)-

I think I've sort of turned into a rambling and incoherent fool, so I will wait for the first post back with some feed back, but I suspect I should just post my question again for clarification-

"Why does temperature affect the Maximum Permitted Zs Values for Protective Devices?"

Thanks in advance- And be gentle, its been a while since I've dealt with the numbers side of things! A little mocking of me is expected here!

Many Thanks-

Andrew M
 12 August 2014 04:36 PM
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AJJewsbury

Posts: 11493
Joined: 13 August 2003

It might help if you explained where you got your figures from - they're confusing me!

My BGB says 2.3 Ohms for a B20 for starters. (Were you looking at a B16?)

Where does 1.16 * 1.12 come from?

- Andy.
 12 August 2014 05:24 PM
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Sanft

Posts: 37
Joined: 22 August 2011

Ahah- Yes I'm most definitely losing it, you are quite right with 2.30!

Also using the complete wrong figures with 1.16 below- but see below explanation

and the 1.16 * 1.12 is in the OSG, with the correction factors, Tables B6 and B8

1.85 (Corrected Value) * 1.12 (Temperature correction factor for 40 Degrees) = 2.07 Ohms @ 40 Degrees is the Max Zs for a circuit installed on a B20 MCB?

Hope this clears things up on the number front- Still scratching my head about it- I'll pop on later when i get home to look at any responses- got a long journey home now!

Thank you for the Response Andy-

Andrew M
 12 August 2014 05:41 PM
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AJJewsbury

Posts: 11493
Joined: 13 August 2003

I've only got an older red OSG to hand, so I'm having to guess a bit. I suspect they're talking about an ambient air temperature of 40 degrees when you take the measurements (rather than a conductor operating temperature).

What you're aiming for is a loop of 2.3 Ohms for a B16.

The copper that would have a resistance of 2.3 Ohms at 70 degrees would have a resistance of 1.85 Ohms at 10 degrees (i.e. it was dead and cold when you tested it)

But if the copper was at 40 degrees (say in a boiler room) when you tested it dead it would have a higher resistance than the tabulated 10-degree resistance - by a factor of 0.004 per degree C. 30 degrees higher than 10 degrees means 1+0.004*30 = 1.12; 1.12 * 1.85 = 2.07 Ohms.

So this same bit of copper has a resistance of 2.3 Ohms at 70 degrees, 2.07 Ohms at 40 degrees and 1.85 Ohms at 10 degrees. So what you're looking for depends on what temperature you're measuring it at.

- Andy.
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