2392 Question: Maximum earth loop impedance for 20A BS88 (gG) fuse

[rustynails]

What is the maximum earth loop impedance to BS7671:2008 for a circuit protected by a 20A BS88 (gG) fuse giving a 5 second disconnection time is:

A: 7.42
B: 2.91
C: 1.84
D: 1.35

The green book says the ln for this type of fuse is 78A p299, that doesnt give you the right answer though.

:confused5:

 

[Silly Sausage]

Think about what units Zs is measured in and all will be revealed!

or look at a certain table in Chapter 4...

 

[NrVolts]

ide say 2.91 ohms pg 49 7671

 

[telectrix]

give that man a coconut.

 

[rustynails]

Sure, it would be handy to have the book in the exam hmmm. Basically I can't remember what the multiplier for this type of fuse is? Eg type c x 10

 

[i=p/u]

Uo/zs so 230/100=3 so close enough

 

[Deleted member 26818]

It's 230V/78A.

 

[Jonny66]

Use the time/current curves in Appendix 3 for each fuse type/rating at the relevant max disconnection time (eg 0.4 or 5 secs). Then divide 230V by this current as above.

 

[i=p/u]

Even better spin, like you preciseness

 

[rustynails]

OK guys thanks for your replies. Just wanted to clear things up a little. I had no idea that a BS88 (gG) fuse was x10 to get the maximum operating time/current. But now I do so :teeth_smile:

 

[Deleted member 26818]

Even better spin, like you preciseness

Do you mean pedanticness?

 

[Sparky Ninja]

The 2392 is closed book, so this question would not crop up.

 

[ackbarthestar]

Are they looking for an approximation or preciseness here since none of the answers given by the OP, so are just approximations.

Zs = 230/78 = 2.95 ohms

* 0.8 for operating temperature adjustments = 2.4 ohms

Unlikely to be a C&G question/Answer

 

[Richard Burns]

Are they looking for an approximation or preciseness here since none of the answers given by the OP, so are just approximations.

Zs = 230/78 = 2.95 ohms

* 0.8 for operating temperature adjustments = 2.4 ohms

Unlikely to be a C&G question/Answer

They are looking for the number in the BRB as per post #3, which is 2.91 ohms

 

[ackbarthestar]

I forgot that we might have been looking at an earlier edition