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Posted By Topic: D type MCB Earth fault loop

Blownfuse
Oct 15 2019 16:44

I have a 3 phase D type 40A MCB for a submain protection therefore it can have the 5 second rating applied, where can I find the 5 second earth fault loop impedance value for MCBs. AS/NZS 3000 only shows 0.4s EFLI values?
   

DougP
Oct 15 2019 17:03

You can calculate it from the trip curve of your MCB.

So if the 5s trip rating of your breaker is 3x rated current..

Then 230/(40x3)= 1.92 ohms
   

OctaneOutlaw
Oct 15 2019 19:30

Wouldn't you not need to test the earth fault loop as it's only required for socket outlets not protected by an RCD to confirm a 0.4s disconnection time and isn't required for submains or anything else? Isn't this why the 5s times are not there because the test isn't a requirement for any of the items that need to trip within 5s?
   

DougP
Oct 15 2019 20:12

It's the same requirement for earth conductor continuity/resistance tests. The value must be low enough to cause the protective device to operate in the required time.

Often we don't even take it into consideration. Because if we size the circuit correctly for voltage drop and have the correct size of earth conductor, the earth resistance and loop impedance will almost always be correct for a 0.4sec disconnect time.
   

AlecK
Oct 16 2019 09:21

The reason there are no "5s" values given for mcbs is not simply because there's no requirement to test; but because with EFLI the device is operating in the "instantaneous" part of the curve, and will be pretty much instantaneous due to the operation being magnetic instead of thermal.

Look at Figs 2.12 to 2.14; and note that part of all the mcb curves is vertical - once the current gets to that level, the operation doesn't get any faster

So if your earth continuity complies with table 8.2, that part of the EFL will also comply for 0.4 s; so there would have to be a REALLY big impedance in the active part (the sort that will show up as a significant operational problem) to bring the overall EFLI up to where it wouldn't operate in 0.4 s let alone 5 s.

For D-curve the instantaneous operation is between 10x to 14x rating, so your 40 A device will be in the instantaneous part of the curve somewhere between 400 A & 560 A, and operating time is less than 1 s.
So unless your PSSC is less than 0.5 kA, you don't have a problem.

Compare a C-curve; where instantaneous operation is between 5.5x & 8.8x In (220 A - 352 A), and operating time in this part of the curve will be less than 2 s.

Whereas fuses are an entirely thermal device,and don't have an "instantaneous" part of the curve; so 5 s values are given.
   

Blownfuse
Oct 17 2019 09:01

This is an existing sub main the MCB need to be changed to a D curve as a welder socket has been installed. When the EFLI test was done it was marginaly greater than required for 0.4s disconnect trip time, as it is a submain it only needed to operate in 5s. I have got the data from the MCB manufacturer and it is now sorted. Thanks for your help.