very low efli

[Lectrician]

This comes up so often it is getting boring.

You can fit a standard DB with 6 or 10ka breakers aslong as the submain is protected by a device with the correct PFC rating and can withstand the let-through-energy.

Search for MCCB's on the MEM website, I think they have a good explanation.

I work in many industrial sites with the sub in the plantroom. Do you seriously think MCCB boards would be required everywhere!!!???!!!

 

[securespark]

I have got impossibly low readings with the Alphatek, I just retested and it came up with a sensible reading.

 

[johnmf]

You can fit a standard DB with 6 or 10ka breakers aslong as the submain is protected by a device with the correct PFC rating and can withstand the let-through-energy.

ive heard the arguments with this one before. believe there are differeing opinions on this.

any views on this.

the lowest rated mcb in the board is 10KA but the pscc at the board is above this (16KA)

is it alowable to fit bs88's to protect the board and then be aloud to use the 10KA breakers even though the KA is above this.

must admit ive never had to explore this problem before.

where do you find information on what let through energy different rating hrc fuses can withstand?

 

[securespark]

Never mind that. Have you double checked the Ze with another piece of kit?

 

[johnmf]

yeah, several times believe me!

the ze is 0.02 but it depends what mood the meters in!

am 100 percent confident that the reading is correct.

 

[securespark]

the ze is 0.02 but it depends what mood the meters in!

Not sure what this means! I appreciate you've checked the Ze more than once, but have you used a different piece of kit to rule out a temperamental meter?

 

[johnmf]

sorry securespark, just having a pop at the meter.

in my opinion the efli testers arent without error and i believe the accuracy isnt always brilliant but it serves its purpose.

the efli has been tested several times with more than one meter and comes up the same.

 

[Adam_151]

any views on this.

How long have you got?

Basically if you haven't got sufficent breaking capacity, then the let through of the upstream device needs to be less than what the downstream device can withstand without damage, let through figures are easy come by, just look at a data shet for a fuse (will give you a constant) or a mechancial breaker (will you you a graph showing let through against the current you through at it) but I'm not sure I've seen figures for what they can stand when the current is higher than what they can break , what you could o though is compare this let-through with the k²s² of the cables downstream of the breakers you are going to assume will not trip

What you you can do, is work in the current cut off properties of the fuse into things (shown on a different graph to let through on the datasheet) and you might be supprised by the amount the fuse actually reduces the current by (google LR85.pdf for an example for BS1361 cut out fuses)

 

[johnmf]

a complicated subject indeed....

thanks for all the input.

my initial thoughts are a merlin 3 phase board with standard 10KA protective devices for the circuits. the KA at the board is going to be 16KA which i know is no good.

not sure where to obtain the relevant info but does anyone have the information whether 3x125A bs88's would be suitable for protecting the db given the discussions in this post.

 

[Spark123]

I found this in the "Square D" loadcentre Catalogue (May 2002);

 

[Lectrician]

It isn't complicate - it is VERY common.

All large industrial sites have a PSC which often off-range with test kit.

As long as the main panel board (or busbar chamber/sw-fuse) arrangement it specced accordingly, downstream DB's etc can usually be derated.

If not - there must be tens of thousands of installations failing on this count.....including houses in large cities where huge subs and huge cables give rise to very low EFL's...........Good job for the 1361 or 88 DNO fuse to provide the neccessary.

 

[Spark123]

Agreed, but imo is not a decision to take lightly and without consulting the manufacturer.