PFC question for 2391 exam

[lee1982]

doing revision for 2391 exam and i cant seem to find the answer to a question, would appriciate any help, i may just be missing the obvious here.

state under what circumstances is it unnecessary to carry out a prospective fault current test at each sub-main distribution board on a instillation.

The question has 3 marks so guessing i need 3 reasons!

 

[dingbat]

doing revision for 2391 exam and i cant seem to find the answer to a question, would appriciate any help, i may just be missing the obvious here.

state under what circumstances is it unnecessary to carry out a prospective fault current test at each sub-main distribution board on a instillation.

Read all of 2.7.16 on pages 53 & 54 of the current GN3, paying particular attention to the very last sentence.

The question has 3 marks so guessing i need 3 reasons!

Not necessarily.

 

[lee1982]

still none the wiser! last sentance on 54 before 2.2.17 phase sequence is this

where the protective devices used at the origin have the necessary rated breaking capacity and devices with similar breaking capacity are used throughout the instillation, it can be assumed that the regulations are satisfied in this respect for all distrobution boards.

 

[dingbat]

still none the wiser! last sentance on 54 before 2.2.17 phase sequence is this

where the protective devices used at the origin have the necessary rated breaking capacity and devices with similar breaking capacity are used throughout the instillation, it can be assumed that the regulations are satisfied in this respect for all distrobution boards.

That's the one.

If you can't work out the answer you really ain't ready... but you've got another 30 hours before the exam.

Surely you can equate this sentence with the question you are puzzling over?

 

[ColJack]

still none the wiser! last sentance on 54 before 2.2.17 phase sequence is this

where the protective devices used at the origin have the necessary rated breaking capacity and devices with similar breaking capacity are used throughout the instillation, it can be assumed that the regulations are satisfied in this respect for all distrobution boards.

just rearrange and negate the last bits..

IF "it can be assumed that the regulations are satisfied in this respect for
all distrobution boards" WHEN "the necessary rated breaking capacity and devices with similar breaking capacity are used throughout the instillation"

THEN "it can be assumed that the regulations are" NOT "satisfied in this respect for
all distrobution boards" WHEN "the necessary rated breaking capacity and devices with similar breaking capacity are" NOT "used throughout the instillation"

 

[ericmark]

It does seem an odd question when one considers fault current and impedance are just different ways of measuring the same thing as long as the voltage remains the same.

So why do we need to know? Two things first to ensure the protective device can handle the prospective fault current. This would not be needed to be measured where the “let through” current of proceeding protective device would limit the prospective current to a value within the limits. I would guess this is the answer they are looking for.

Second to ensure the magnetic part of the protective device will have enough current if a short circuit occurs to open the device. This I see as a problem as although between line and earth the need to keep to within the limits may not be required when a RCD is used one would still want to ensure with a line neutral fault the device will trip.

For some reason when looking at the overload device protection we tend to look at the amps. For device operation we tend to look at the ohms. In real terms for example a B32 MCB we know will need 5 x 32A to trip which is 160A since we normally do first checks without power we convert that to ohms so 230/160 = 1.4375 which rounds up to the 1.44 found in BS7671:2008.

We would normally measure the PFC/Loop Impedance twice first between line and earth and second between line and neutral. With the former even with a fuse limiting the “let through” amps and a RCD limiting the required impedance required to trip we still need to ensure it is a value exceeding 200 ohms (1.15 Amps) to be may not be stable. And with the latter we still need to ensure a current high enough to cause the device to trip.

However the exam it seems is written by using the English and no consideration is given to the mathematical or logistical relationships. We see this in questions about Skilled and Competent where looking at the definition a Competent person is Skilled but not vice versa so questions can be worded so either Skilled or Competent could be correct answer. And we have to forget our knowledge as an electrician and answer purely by looking for the “and others” phrase.

My Guidance Notes 3 is well out of date but I would suspect there is some phrase I would think the answer is contained within:-
536.1 General
Where co-ordination of series protective devices is necessary to prevent danger and where required for proper functioning of the installation, consideration shall be given to selectivity and/or any necessary back-up protection.
Selectivity between protective devices depends on the co-ordination of the operating characteristics of two or more protective devices such that, on the incidence of fault currents within stated limits, only the device intended to operate within these limits does so (see Regulation 536.5).
The rated breaking capacity of a protective device shall be not less than the maximum prospective short-circuit or earth fault current at the point at which the device is installed unless back-up protection is provided. A lower breaking capacity is permitted if another protective device (a back-up protective device) having the necessary breaking capacity is installed on the supply side and the characteristics of the devices are suitably co-ordinated such that the energy let-through of the upstream device does not exceed that which can be withstood without damage by the downstream device (see Chapter 43).
NOTE: Back-up protection on the load side of a protective device is acceptable only if the risk of a fault in the circuit between the two protective devices in series is negligible.

And also in:- Back-up protection. Protection which is intended to operate when a system fault is not cleared, or abnormal condition not detected, in the required time because of failure or inability of other protection to operate or failure of the appropriate circuit-breaker(s) to trip.

Plus of course the GN3 reference already given. I would be interested to see that reference but not enough to buy the book.

 

[ericmark]

still none the wiser! last sentence on 54 before 2.2.17 phase sequence is this

where the protective devices used at the origin have the necessary rated breaking capacity and devices with similar breaking capacity are used throughout the instillation, it can be assumed that the regulations are satisfied in this respect for all distribution boards.

This confirms what I had considered all to do with "let through" amps. If the amps can't exceed the value because of the previous device then of course you don't really need to measure. Or at least switch the meter to measure in amps. If anything like my meter once measured one can flick switch to display in ohms or amps so you don't need to calculate it's all done in the meter.

Still think badly worded question as in real terms you still need to measure to ensure the ELI is within the limits.

 

[lee1982]

dont think im ready for the exam myself, its tomorow at 6 (not sure how u know roughly ), though i will give it my best! think a break has helped been at it since 7.

if the lowest protective device has a breaking capacity higher than the PSC at the origin then you dont have to take measurements at other points.

god help me and hope i get lucky with the questions i get!

 

[Risteard]

PFC.

No such thing as PSC.

PSCC is Prospective Short-Circuit Current; PEFC is Prospective Earth Fault Current. Prospective Fault Current is the highest of the two.

 

[dingbat]

dont think im ready for the exam myself, its tomorow at 6 (not sure how u know roughly )

It should be at 18:30. I know because I teach it.

if the lowest protective device has a breaking capacity higher than the PSC at the origin then you dont have to take measurements at other points.

That's just about it, but do be careful of your terminology. Best of luck.

 

[lee1982]

dont think im ready for the exam myself, its tomorow at 6 (not sure how u know roughly )

It should be at 18:30. I know because I teach it.

if the lowest protective device has a breaking capacity higher than the PSC at the origin then you dont have to take measurements at other points.

That's just about it, but do be careful of your terminology. Best of luck.

im gonna need alot of luck! i know my terminology will let me down! thanks for the input, i have to be there for 6 so prob will start at 6.30!

 

[FDJ]

PFC.

No such thing as PSC.

PSCC is Prospective Short-Circuit Current; PEFC is Prospective Earth Fault Current. Prospective Fault Current is the highest of the two.

PFC=Power Factor Correction since about 1930

Oh! those re-use of acronyms

Frank

 

[LoveRocket]

PFC.

No such thing as PSC.

PSCC is Prospective Short-Circuit Current; PEFC is Prospective Earth Fault Current. Prospective Fault Current is the highest of the two.

PFC=Power Factor Correction since about 1930

Oh! those re-use of acronyms

Frank

Don't even get me started on how the air blowing lot have hijacked High Voltage Alternating Current...

 

[B67BU]

dont think im ready for the exam myself, its tomorow at 6 (not sure how u know roughly )

It should be at 18:30. I know because I teach it.

if the lowest protective device has a breaking capacity higher than the PSC at the origin then you dont have to take measurements at other points.

That's just about it, but do be careful of your terminology. Best of luck.

im gonna need alot of luck! i know my terminology will let me down! thanks for the input, i have to be there for 6 so prob will start at 6.30!

So how did your exam go?