HIGH PEFC??

[Sidewinder]

NO Chicki,

The mcb may not go first and even if it did it is only rated at 6kA it would "see" 11.1kA in your scenario.

What would that do to it etc.?

Why do they put those ratings on mcb's?

What do they stand for, what do they mean?

 

[Chr!s]

NO Chicki,The mcb may not go first and even if it did it is only rated at 6kA it would "see" 11.1kA in your scenario.

What would that do to it etc.?

Why do they put those ratings on mcb's?

What do they stand for, what do they mean?

Not sure i would agree with thatstatement

 

[m4tty]

Can you explain why please chris as I'd like to learn from yous. Cheers

 

[steptoe]

You could have a fault current of 16kA matty, now, you and CHICKI need to investigate what could/may happen if this were the case, remember, your MCB is rated 6kA, why, what bearing may this have and why wouldn't the MCB trip as soon as the current reached the MCBs OC rating?

---------- Post Auto-Merged at 11:13 ---------- Previous post was made at 11:11 ----------

Chris, that may, or may not happen, its about trying to get the OP to realise the significance of kA ratings on OCPDs, in relation to PFC.

 

[ianmacd]

I think it's covered by the ka rating of the mainfuse as said above mate. There's a reg but I can't find it at the mo

Try 434.5.1 Matty.

---------- Post Auto-Merged at 19:15 ---------- Previous post was made at 19:11 ----------

its about trying to get the OP to realise the significance of kA ratings on OCPDs, in relation to PFC.

Not just OCPD's but any isolating device.

 

[gordy71]

Great thread by the way guys, my guess is a fault of this size (10+ ka) would damage the mcb and blow the 1361. i say this because the fault is so great, the fault would have just enough time to blow the 1361 before the mcb blows up. What i mean is the disconnection time would be so short, the current would reach the 1361 for enough time for it to blow. Or my next guess is that the fault wouldnt even reach the mcb as the fault current being drawn into the house would be stopped by the main fuse blowing? Just a stab in the dark as i dont have any books with me.

 

[ianmacd]

Great thread by the way guys, my guess is a fault of this size (10+ ka) would damage the mcb and blow the 1361.

Thats most likely an understatement. Lets hope its not one of the recalled devices that had failing arc chambers.

 

[steptoe]

Try 434.5.1 Matty.---------- Post Auto-Merged at 19:15 ---------- Previous post was made at 19:11 ----------

Not just OCPD's but any isolating device.

you are of course correct,

it was just aimed more at the OPs question, but yes, it does apply to all devices in the system where a kA rating is relevant.

---------- Post Auto-Merged at 19:26 ---------- Previous post was made at 19:23 ----------

Great thread by the way guys, my guess is a fault of this size (10+ ka) would damage the mcb and blow the 1361. i say this because the fault is so great, the fault would have just enough time to blow the 1361 before the mcb blows up. What i mean is the disconnection time would be so short, the current would reach the 1361 for enough time for it to blow. Or my next guess is that the fault wouldnt even reach the mcb as the fault current being drawn into the house would be stopped by the main fuse blowing? Just a stab in the dark as i dont have any books with me.

but why?

and how would it happen,?

IIRC,

there was a thread about this some time ago, and what would happen in this type of scenario.

so,,,,

if you dont want to trawl through your notes and books,

you can trawl through this forum and find the reasons.

unfortunately I cannot recall just how long ago, or the title, of said thread. sorry.

 

[ianmacd]

but why?

and how would it happen,?

IIRC,

there was a thread about this some time ago, and what would happen in this type of scenario.

so,,,,

if you dont want to trawl through your notes and books,

you can trawl through this forum and find the reasons.

unfortunately I cannot recall just how long ago, or the title, of said thread. sorry.

When you open an isolator, they should have an arc chamber to catch any plasma flash. That will only have been designed to 6kA if thats what the device is designed for. If there was a high current running though an isolator when it tried to open then the arc chamber may fail (or at least be irrevocably damaged) and the MCB should be replaced. Depending on the failure you may be picking parts of the mcb up from all over the place.

 

[steptoe]

When you open an isolator, they should have an arc chamber to catch any plasma flash. That will only have been designed to 6kA if thats what the device is designed for. If there was a high current running though an isolator when it tried to open then the arc chamber may fail (or at least be irrevocably damaged) and the MCB should be replaced. Depending on the failure you may be picking parts of the mcb up from all over the place.

short answer,

No,

but although you are along the right lines iun one instance,

thats not entirely correct as the kA rating applies to much more than simply isolators, or even MCBs,

consider 3036 fuses for instance, or even 1361s.

 

[kme]

Referring to Sidey`s comment on page 2; BSEN 60947-3 annex ZA was the reference. It allows for just such a scenario, and IS in the BRB

KME

 

[ianmacd]

short answer,No,

No to what? That IS how an MCB works. Other circuit breakers may use fluids or a vacuum in the arc chamber but MCB's rely on the arc chamber to contain the plasma.

 

[steptoe]

No to what? That IS how an MCB works. Other circuit breakers may use fluids or a vacuum in the arc chamber but MCB's rely on the arc chamber to contain the plasma.

I know how an MCB works,

you have'nt said what happens when a device of a particular kA rating is faced with a fault of a greater kA rating.

you have just said the plasma shield fails, how did this occur?

do 3036s have these fitted? they will re-act in the same way.

you have simply said how an MCB works, in part.

how does it fail? how would a 3036 fail in the scenario? or any other device?

the answer to this is exactly the same(virtually) for any device that is faced with a fault in excess of its kA rating.

 

[ianmacd]

I know how an MCB works,you have'nt said what happens when a device of a particular kA rating is faced with a fault of a greater kA rating.

you have just said the plasma shield fails, how did this occur?

do 3036s have these fitted? they will re-act in the same way.

you have simply said how an MCB works, in part.

how does it fail? how would a 3036 fail in the scenario? or any other device?

the answer to this is exactly the same(virtually) for any device that is faced with a fault in excess of its kA rating.

I thought that was obvious. I guess not. More current, bigger arc. Extremely hot ironised gas of a higher magnitude than the arc chamber is designed for and can fail. Thats assuming it can actually open and isn't welded closed and just melts because of the fault current.

 

[steptoe]

I thought that was obvious. I guess not. More current, bigger arc. Extremely hot ironised gas of a higher magnitude than the arc chamber is designed for and can fail. Thats assuming it can actually open and isn't welded closed and just melts because of the fault current.

that is more like a description of the fault,

remember ian, this is for the benefit of a lot of people, OP included, that do not realise the whys and wherefores of what kA ratings are for.

BTW, its the ionisation that is the issue.

do you want to enhance,? or let the OP see if he has a grasp of this? or someone else for that matter?

 

[Sidewinder]

ian,

I am trying to help the OP get his head around this, so I'm not going to answer any of your points just ask more questions.

What is the problem if the mcb fails in the welded shut position anyway?

What is the problem if the mcb is damaged beyond use?

Remember mcb's are a disposable item anyway, they are only designed to take so many fault current disconnections before they must be replaced, the exact number is definedin the sandards as a minimum.

If they take a full rated current disconnection then they are suposed to be binned.

Thing is it is difficult to tell how many times they have tripped and at what current levels!

 

[CHICKI]

why do they put the KA ratings on the breakers if there isn't really any point?

---------- Post Auto-Merged at 17:23 ---------- Previous post was made at 17:17 ----------

some bloke just told me that changing the system would help the PSCC due to parallel paths with the previous TNCS system or some **** like that?? impedance of the ground, blabla, because the transformers quite close, blabla!!??

 

[kme]

You would lower the earth fault path; but not the PSCC - FCOL, the TNC-S earth IS the neutral; so the only bit of parallel path you would remove is between the HED and the CU - however, it isn`t a TRUE parallel path, as, at the CU end, one of the conductors is a neutral; the other is an earth. TT won`t alter the live-neutral fault current, as you aren`t (and can`t) do anything with the live or neutral conductors ( other than lengthen them......).

KME

 

[Sidewinder]

why do they put the KA ratings on the breakers if there isn't really any point
They are there because the breakers are required to have a maximum breaking capacity.

That is they must be capable of interrupting a fault current up to and including their rated maximum.

They do no have to be reusable after this situation.

They must be able to do this where ever they are fitted in an installation, with one or two exceptions.

Your scenario appears from the info you have given to be one of those exceptions.

As per the reference that KME posted previously.

some bloke just told me that changing the system would help the PSCC due to parallel paths with the previous TNCS system or some **** like that?? impedance of the ground, blabla, because the transformers quite close, blabla!!??

My red above also.

Changing to a TT or otherwise artificially increasing the earth fault loop impedance will reduce the L-E PFC, i.e. PEFC, however it will not reduce the L-N PFC, ie the PSCC.

In a different, say industrial scenario where you fit a 60898 breaker in a downstream db then you will probably find that you cannot per ce disregard the fault current relationship.

However, as I gather this is a domestic CU mounted a few m from the meter etc?

 

[Chr!s]

why do they put the KA ratings on the breakers if there isn't really any point?

Why do you think theres no point?

Uk Annex ZA is used where specific requirements are met, its mainly used in the domestic scenario. Where Annex ZA does not apply then Reg 434.5.1 will need to be met.