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Topic Title: BS88 HRC Fuses
Topic Summary: British Standard BS 88-1:2007
Created On: 27 November 2007 01:43 PM
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 27 November 2007 01:43 PM
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SpitFire1982

Posts: 7
Joined: 27 November 2007

Hi,

This is my 1st post on the IET forum.

I have some questions regarding BS88 HRC fuses. According to the British Standard BS 88-1:2007; a "gG" class fuse at rated voltage of 550V ac (or 415V ac) has a breaking capacity of 80kA. How long will the fuse be able to withstand such an excessively high fault current?

If you take a 63A rated fuse as an example: According to table 7 of the standard, the fuse has a pre-arcing I²t(MIN) value at 0,01s is 9 000 A²s and a I²t(MAX) value at 0,01s is 27 000 A²s. If my calculation is correct, this corresponds to current of the values 948A (0.01 seconds) and 1643A (0.01 seconds) respectively. This is way below the 80kA breaking capacity specified!!!

The way I understand it is that for a fault current of 800A for 0,01s the fuse will operate but not blow or rupture. What would happen to a fault current that falls within the min and max I²t for the same time??? What is the relevance of the 80kA breaking capacity then???

I tried to contact different fuse manufacturers with no success. The all refer me to their relevant fuse data sheets, and seeing that I don't understand the 80kA rating, pre-arcing min and max value, I am unable to use the data sheets effectively.

I hope there is someone out that can help me clarify this issue of mine

Regards
Bernard
 27 November 2007 02:50 PM
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OMS

Posts: 19602
Joined: 23 March 2004

I think you are confusing breaking capacity with the fault current necessary to operate the fuse in a given time.

Taking your example whilst the 63A fuse may only require 710A to operate in 0.1 of a second, it may be exposed to a fault level much higher than this (dependant on where it is situated in a system

Eg - you may use a BS 88 fuse assembly to protect say an external lighting supply which is electrically close to say the main switchboard of a large hospital where it may be exposed to a fault level some two orders of magnitude greater than the 710A necessary for 0.1 second disconnection.

Not sure if that answers your question without confusing the issues of I2t let through energy - you are moving into the territory of back up protection and recognition of the real let through energy as opposed to the prediction based on simple analysis

Regards

OMS

-------------------------
Failure is always an option
 27 November 2007 04:19 PM
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mstaple

Posts: 324
Joined: 23 July 2004

Off the top of my head (I will check this tonight), the breaking capacity is the fuses abiltiy to withstand a fault current without exploding. So the BS88 fuse can withstand a fault current of 80kA without completely disintegrating (i.e the fault is contained within the fuse itself).

How long the fuse will be able to withstand this current (if you can achieve it) is derived from the fuse curve. The lower the fuse rating, the quicker it will internally disintegrate. (Inside the BS88 fuse body is the fuse wire/wires which are surrounded by sand. The fuse clears the fault once the wire/wires are melted inside the sand).

So the fuse is melted by energy which is the I2T number you are referencing. A fuse will melt if overloaded for sufficient time to heat and melt, the higher the current, the quicker it is heated and melts, so the quicker the operation.

So why is the breaking capacity important? Well we don't want the fuse exploding under fault conditions and killing someone. When you calculate the fault impedance for your fault current, you should use this to check that the fuse removes the fault in the correct disconnection time AND that the circuit can withstand the fault current (this is not always checked)

If you can get hold of a ruptured BS88 fuse in DIN format that is large (about 300A), you can take them apart. The two endplates unscrew from the ceramic body which releases the sand. Between the two endplates are wafer thin copper strips which is the fuse wire itself. These are the parts which melt and fuse with the sand to make glass. This extinguishes the arc but keeps the 'flash and bang' contained safely in the body.

Edited: 28 November 2007 at 01:32 PM by mstaple
 27 November 2007 06:30 PM
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OMS

Posts: 19602
Joined: 23 March 2004

When you calculate the earth impedance for your fault current, you should use this to check that the fuse removes the fault in the correct disconnection time AND that the circuit can withstand the fault current (this is not always checked)


Not wanting to complicate the issue but the fault level analysis (and hence determination of the device breaking capacity) is usually undertaken on the highest fault level likely to be achieved at that point in the distribution. An earth fault is likely to be considerably less than say a bolted 3 phase symetrical fault. (Although determination of the earth fault current is required for other reasons)

Regards

OMS

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Failure is always an option
 28 November 2007 01:33 PM
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mstaple

Posts: 324
Joined: 23 July 2004

Good point OMS and your absolutely correct. That wasn't bad for off top of my head though!

I did find an old book last night so have some better definitions for when you talk to the fuse companies Spitfire

Prospective Fault Current - the is the theoretical maximum value of current that could flow in a circuit with no circuit protection. This is supply voltage/impedance of the fault loop. For a single phase system this is typically about 230/0.1 = 2300A. For a 3 phase system the highest value is a phase to phase fault (OMS point) which is typically 415/0.1 = 4150A. The closer a fault in a system is to the supply, the less the impedance of the fault loop and the greater the prospective fault current (as the impedance of the fault loop approaches 0, the prospective fault current approaches infinity) Therefore as we get near the main transformer the prospective fault currents become very large and this is where we need to ensure circuit protection can withstand the prospective fault current.

Breaking Capacity - This is the maximum theoretical value of prospective fault current that the circuit protective device can clear safely (i.e. without exploding) The breaking capcacity should always be higher than the calculated prospective fault current.

Pre-Arcing time (I2T min) - this is the point at which the heating effect of the current flowing through the fuse element is sufficient to start melting the fuse. We sometimes call this 'stressing' the fuse. If you 'stress' a fuse long enough or many times it will eventually fail, so acts as an overload protection

Arcing time - once you reach the pre-arcing time, the fuse begins to arc and melt the fuse. The arcing time is complete once the fuse is melted or the current flowing through the fuse returns below the pre-arcing value.

Pre-Arcing time + Arcing time (I2T max) - the pre-arcing time plus the arcing time is the total clearance time of a fuse

If you can find any bullrush curves of fuses this is a nice representation of these fuse characteristics. For discrimination in a supply system, the sub-fuse must clear the fault before the pre-arcing time of the main fuse is reached otherwise the main fuse will be pre-stressed and could operate when not required.

The clearance time of a fault is defined in the regs but you must also consider the load you are protecting. gG is for general loads, gM and aM are for motors and gL for semiconductors. So gM and aM can withstand long overcurrents before reaching pre-arcing for motor starting currents and gL reaches pre-arcing very quickly to prevent damage to semiconductors.

Hope that helps, coz it took ages to write!

Edited: 28 November 2007 at 02:05 PM by mstaple
 28 November 2007 02:01 PM
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SpitFire1982

Posts: 7
Joined: 27 November 2007

Thanks mstaple

Thing are starting to make sense now!!! Let's take the following "Bussmann" BAO63 fuse as an example (the link provided below):

Bussmann BS88 BAO63 Fuse

The BAO63 fuse has an ampere rating of 63A. The pre-arcing I²t is given as 6700 and the total I²t at 18800 (at rated 415V ac). For the pre-arcing value given (as 6700), at what time and current would it equal to 6700 (is this given as a fixed time like at 0,01s). Also, for the total I²t value given (as 18800), at what time and current would it equal to 18800 (is this given as a fixed time like at 0,01s). From your definition, I deduce that the total I²t is the pre-arcing I²t + the arcing I²t. When selecting a fuse, which factor is more important the pre-arcing I²t or the total I²t???

Thanks for you help thus far

SpitFire1892
 28 November 2007 02:27 PM
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mstaple

Posts: 324
Joined: 23 July 2004

No problem.

I think your confusion lies in the fact that you are trying to work this out around the wrong way so it's a bit difficult to answer your question. The curve represents I2T to rupture the fuse i.e. current x current x time. For the same value of I2T we could have a high current and short time or a low current and long time. Any point on the curve represents the same total I2T (18850) value for that particular time and current.

In effect, what the curve represents is that the fuse can withstand low overcurrents for long time and high overcurrents for short times.

Normally we calculate the prospective short circuit current, find this on the x-axis. Then draw a line up to the fuse curve for your chosen fuse. At the point which the line meets the fuse curve, draw a line across to the y-axis and you have you disconnection time. The curve is therefore showing the total I2T time to rupture the fuse and this is the most important number. I think this is 5 seconds for a fixed installation and 1 second for portable appliances.

Hope that explains bit better
 28 November 2007 02:34 PM
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mstaple

Posts: 324
Joined: 23 July 2004

Sorry forgot to say, the pre-arcing time is important to achieve discrimination between main and sub fuses in a system. You really need Bullrush Curves for this but not sure where to get them. Generall rule of thumb is the primary fuse must be double the rating of the sub fuse until you get to large sizes >300A.

For a ring main on a house, the sub fuse max rating is 13A so the main fuse must be at least double this to prevent nuiscance activation of the main fuse, hence 32A I think. If it was lower than this, it becomes a race between the two fuses as to which one operates in a fault. A small local fault may take the whole ring out which is not good.
 28 November 2007 05:07 PM
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SpitFire1982

Posts: 7
Joined: 27 November 2007

Things are slowly but surely making sense!!! I had difficulty relating the pre-arcing I²t and total I²t values to the curve, but now it makes sense.

Let's take that BA0 63 fuse again as an example:

The way I now understand it is that the fuse will safely clear/operate (but not rupture or explode) within the pre-arcing I²t and total I²t value region (6700 and 18800 respectively). What would happen to a potential fault level/current of an I²t value below the pre-arcing I²t value of 6700 (lets say the potential fault level/current of an I²t value equal to 5500)??? I expect that the fuse will continue as normal and not melt the fuse element. At the other end of the spectrum, what would happen to a potential fault level/current of an I²t value above the total I²t value of 18800 (lets say the potential fault level/current of an I²t value equal to 25000)???

Thanks for you help!!!

Regards
SpitFire1982
 28 November 2007 05:52 PM
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mstaple

Posts: 324
Joined: 23 July 2004

No problem mate.

The fuse will always clear/operate when the total I2T value is reached.

If the current does not reach the pre-arcing value the fuse will not be affected. A 10A rated fuse can take 10A of current for almost infinity time as the heating effect (I2T) is not sufficient to affect the fuse. The fuse only ruptures once sufficient energy (I2T) has been passed through it. This could be many overload currents taking the fuse in and out of pre-arcing but the progressive deteriation due to the heating effect of the current (sum of the I2T overloads) leads to failure of the fuse element.

Or this could be a single high current (short circuit value greater than total I2T value) causing rapid heating to melt the element. This will destroy the fuse in a fast time.

The rating of the fuse determines the energy it can pass while dissipating the heat sufficiently to not cause deteriation to the fuse. Deteriation to the fuse, pre-arcing, normally starts around 1.5 times the rating. This is the level where the fuse cannot dissipate the heat sufficiently so the element temperature keeps rising untill it melts.
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