Hi has anyone had the please of finding a high kA reading on a job? It's single phase, BS 88 fuse, 440V & 100A.First test got a reading of 15.8kA! Didn't like what I saw so tried again and again and again - eventually went to 9.3kA but the next time went up to 15.8kA again! Any possible reason for the fluctuation? Tester seems to be okay as it was only calibrated a year ago. Also calibrate it myself every month on a designated ciruit. The client says there is a transformer/"power station" half a mile down the road and that keeps blowing. I am changing the mcb for the circuit I am working on to one with a kA rating of 16, but apparently don't have to change the others - surely this is not good as I am leaving the installation in a more dangerous position? Any thoughts? Thanks.
High kA reading
- Thread starter 69er
- Start date
Sponsored Links
hmm i though mcbs stopped at 10KA and you needed to go to MCCBs or cartridge fuses for higher breaking capacities.
the rec fuse is there as back up protection anyway in the unlikely event of a fault that make the full PSCC flow and the mcb failing to break it.
the rec fuse is there as back up protection anyway in the unlikely event of a fault that make the full PSCC flow and the mcb failing to break it.
For a single phase supply (I presume 230V to neutral ? - just the fuse is rated at 440V max?) this is indeed a very low reading - the building spur must be a very short run indeed to the street main.
15KA is 15milliohms, 9KA is abut 25milliohms. The variable readings are only 10miliohms apart - could that be down to exact contact positions and pressure applied?
Fortunately, for a fault to really cause this much current to flow it would have to be within a few metres of the MCB, (1mm cable is 18 mili-ohms per metre for example so a lighting circuit fault of a perfect short 50cm (18") from the CU would roughly halve the short circuit current relative to the number you measured - being a 1m round trip).
For higher current circuits with thicker cable this safe distance is correspondingly longer. So most faults, evn if perfect short, but not right next to the board will not involve currents >> 10kA that might weld up the MCBs.
However, as somewhat wryly observed, even if the MCB does welds 'on', the company fuse will operate pretty smartish at that current. In this sense it is more of a theoretical risk than a real one. If in doubt check that the let-through energy of the main fuse will not vaporise the faulty circuit but leave the power on (the I2T sum.) If you are really worried about not blowing the company fuse as a "last ditch" defence then cartrige fuses may be in order for some circuits (or 3m long meter tails to raise the supply impedance a bit! - not a serious suggestion)
15KA is 15milliohms, 9KA is abut 25milliohms. The variable readings are only 10miliohms apart - could that be down to exact contact positions and pressure applied?
Fortunately, for a fault to really cause this much current to flow it would have to be within a few metres of the MCB, (1mm cable is 18 mili-ohms per metre for example so a lighting circuit fault of a perfect short 50cm (18") from the CU would roughly halve the short circuit current relative to the number you measured - being a 1m round trip).
For higher current circuits with thicker cable this safe distance is correspondingly longer. So most faults, evn if perfect short, but not right next to the board will not involve currents >> 10kA that might weld up the MCBs.
However, as somewhat wryly observed, even if the MCB does welds 'on', the company fuse will operate pretty smartish at that current. In this sense it is more of a theoretical risk than a real one. If in doubt check that the let-through energy of the main fuse will not vaporise the faulty circuit but leave the power on (the I2T sum.) If you are really worried about not blowing the company fuse as a "last ditch" defence then cartrige fuses may be in order for some circuits (or 3m long meter tails to raise the supply impedance a bit! - not a serious suggestion)
Hi
Plugwash - they are mcbs that I am using - had to order them in especially - £30 carriage!!! I understand that the rec fuse will provide back up should such a fault occur but then there is a risk of the mcb's being damaged - I ask myself is it worth the risk. I think for the 15 mins or so it's gonna take to change the mcbs I may as well and bee 100% safe - therefore there are no comebacks to me. As I am niceic registered (groan groan) I spoke to them about it to confirm the course of action to undertake and they quoted a page from the OSG for me which states mcb kA must rate up to or including kA at point of origin.
Mapj1 - yes it is a single phase supply 230V to neutral. I can not see the transformer anywhere nearby - client siad its half a mile down the road & keeps "blowing"? Power cuts. As I have ordered the mcbs we will use them - like I said it's better to be safe than sorry - I don't wanna take the risk after seeing some of the wiring. Some cowboy (as the client called him) has run a new ring in the new conservatory connecting it into a socket in the adjacent room (kitchen). Unfortunately they didn't check the circuit properly and have succeeded in running a ring half way through a spur on a spur - oh happy days! Client has a newish fuse board done by a qualified spark ( 2 years ago) - when I opened it I wanted to cry - you have to disconnect 80% of circuits to get to neutral and earth bars! The spark was also colour blind but checked all colours are in correct places - and they are! Thanks for the detailed explanation - question - why is 15kA 15millohms but 9ka 25milliohms? What is the equation for working it out? Me thinks I have asked a stupid question but hey! Thanks again.
Plugwash - they are mcbs that I am using - had to order them in especially - £30 carriage!!! I understand that the rec fuse will provide back up should such a fault occur but then there is a risk of the mcb's being damaged - I ask myself is it worth the risk. I think for the 15 mins or so it's gonna take to change the mcbs I may as well and bee 100% safe - therefore there are no comebacks to me. As I am niceic registered (groan groan) I spoke to them about it to confirm the course of action to undertake and they quoted a page from the OSG for me which states mcb kA must rate up to or including kA at point of origin.
Mapj1 - yes it is a single phase supply 230V to neutral. I can not see the transformer anywhere nearby - client siad its half a mile down the road & keeps "blowing"? Power cuts. As I have ordered the mcbs we will use them - like I said it's better to be safe than sorry - I don't wanna take the risk after seeing some of the wiring. Some cowboy (as the client called him) has run a new ring in the new conservatory connecting it into a socket in the adjacent room (kitchen). Unfortunately they didn't check the circuit properly and have succeeded in running a ring half way through a spur on a spur - oh happy days! Client has a newish fuse board done by a qualified spark ( 2 years ago) - when I opened it I wanted to cry - you have to disconnect 80% of circuits to get to neutral and earth bars! The spark was also colour blind but checked all colours are in correct places - and they are! Thanks for the detailed explanation - question - why is 15kA 15millohms but 9ka 25milliohms? What is the equation for working it out? Me thinks I have asked a stupid question but hey! Thanks again.
Sponsored Links
mapj1 said:
Could you explain what that is please mike, is it something to do with the adiadiabetic equation (though I have only heard of that used in working out minimum cpc csa s)
69er said:
V = IR, I = V/R
Your PSCC tester doesn't actually test this (would get very hot if it did, lol), it measures the resistance, by doing the same test as for Ze, but doing it between phase and neutral, and then calculating current from that
As sometimes occurs, my brevity and arm waving style returns to haunt me.. 
OK, The first one is ohms law with a bit of rounding - Adam spotted that, sums done in head tho so nearest kiloamp will do - apologies for the sloppiness!
The I2T is the adiabatic equation re-jigged for fridays.
One assumes that no heat can get out of the wire (fully lagged - adiabatic) as in the limit of an extremely short heat-up period this is true, as there is no time for heat to spread to the surrounding insulation.
Then one asks, how much energy raises the copper to boiling (for a fuse, or a 'danger temperature' for conductors where damage such as setting fire to the wallpaper becomes likely.)- this is the specific heat times the temperature rise needed, and is a certain number of joules per gram of copper. Now we need to relate the number of grams of copper per unit length of wire, to its heating. The heating is just power times pulse duration time, so I*V*T, when V is volts drop per unit length, while carrying fault current I.
Now if we pretend R per unit length is constant, and eliminate V, we get our old friend I*I*R*T = constant for any particular wire cross-section.
See the tables.
This for a fuse is the pre-arc energy - = defines the maximum current spike before parts of the fuse start to melt. (or for a given current, fixed by the substation characteristics, defines the shortest time to blow, with a zero ohm fault)
For a final circuit, one usually assumes a different final temperature (lower) and so the maximum amount of energy allowed is much less. However, for a fuse there is a second figure - the 'let through energy' in the same units, of I2T, which is the largest transient that will be ever let through to downstream devices during a fault before the fuse has finished breaking the supply.
If one wishes to cascade 2 fuses, and guarantee the thinner one only blows, under any combination of fault conditions, the pre-arc energy of the first, should be greater than the let through energy of the lighter one, so the lighter one has definitely blown clear, well before the big one reaches melting temperature. In your case it is the other way round - you want the fuse to clear before the final circuit conductors set fire to the house. However, the method is the same.
Hope that helps without being too tutorial.
OK, The first one is ohms law with a bit of rounding - Adam spotted that, sums done in head tho so nearest kiloamp will do - apologies for the sloppiness!
The I2T is the adiabatic equation re-jigged for fridays.
One assumes that no heat can get out of the wire (fully lagged - adiabatic) as in the limit of an extremely short heat-up period this is true, as there is no time for heat to spread to the surrounding insulation.
Then one asks, how much energy raises the copper to boiling (for a fuse, or a 'danger temperature' for conductors where damage such as setting fire to the wallpaper becomes likely.)- this is the specific heat times the temperature rise needed, and is a certain number of joules per gram of copper. Now we need to relate the number of grams of copper per unit length of wire, to its heating. The heating is just power times pulse duration time, so I*V*T, when V is volts drop per unit length, while carrying fault current I.
Now if we pretend R per unit length is constant, and eliminate V, we get our old friend I*I*R*T = constant for any particular wire cross-section.
See the tables.
This for a fuse is the pre-arc energy - = defines the maximum current spike before parts of the fuse start to melt. (or for a given current, fixed by the substation characteristics, defines the shortest time to blow, with a zero ohm fault)
For a final circuit, one usually assumes a different final temperature (lower) and so the maximum amount of energy allowed is much less. However, for a fuse there is a second figure - the 'let through energy' in the same units, of I2T, which is the largest transient that will be ever let through to downstream devices during a fault before the fuse has finished breaking the supply.
If one wishes to cascade 2 fuses, and guarantee the thinner one only blows, under any combination of fault conditions, the pre-arc energy of the first, should be greater than the let through energy of the lighter one, so the lighter one has definitely blown clear, well before the big one reaches melting temperature. In your case it is the other way round - you want the fuse to clear before the final circuit conductors set fire to the house. However, the method is the same.
Hope that helps without being too tutorial.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
