H rating on fuses - Page 1
Forum section: electronics / projects

author: Northy
author_rank: Regular Contributor
timestamp: January 19, 2021, 10:05:58 am
content: Hi all,I thought I understood fuse ratings, but maybe nothttps://www.swe-check.com.au/pages/learn_fuse_markings.phpH = High Breaking CapacityWhen would you need to use a H rated fuse?Thanks,G

author: trobbins
author_rank: Frequent Contributor
timestamp: January 19, 2021, 10:31:06 am
content: When connecting to a power source that has a high prospective fault current.

author: Northy
author_rank: Regular Contributor
timestamp: January 19, 2021, 02:13:54 pm
content: Quote from: trobbins on January 19, 2021, 10:31:06 amSorry, I don't understand what you mean?G

author: themadhippy
author_rank: Super Contributor
timestamp: January 19, 2021, 03:29:03 pm
content: Quoteits the amount of current that can flow in the event of a short circuit.whilst the fuse may be rated for 10A the amount of current the supply can provide is much greater,so the fuse needs to be able to safely handle the amount of current the supply can deliver until it decides to blow.

author: Northy
author_rank: Regular Contributor
timestamp: January 19, 2021, 03:34:31 pm
content: Ah, ok, thanks, that makes sense.So how do I work out what I need? Is there guidance/a standard somewhere?Thanks,G

author: wraper
author_rank: Unknown
timestamp: January 19, 2021, 03:36:31 pm
content: https://www.schurter.com/content/download/676506/13549060/version/3/file/Guide_to_Fuse_Selection.pdfQuote

author: temperance
author_rank: Super Contributor
timestamp: January 19, 2021, 05:25:34 pm
content: A more practical example:in some instrial electrical installations they sometimes add extra cable length to guarantee that the short circuit current will be below the H rating of the fuse.

author: Northy
author_rank: Regular Contributor
timestamp: January 19, 2021, 05:38:23 pm
content: The Schurter document is interesting, but how do I know what breaking capacity I need for a mains powered domestic device?Thanks,G

author: rstofer
author_rank: Super Contributor
timestamp: January 19, 2021, 05:55:53 pm
content: Quote from: Northy on January 19, 2021, 05:38:23 pmPerhaps the residence has a breaker panel and you can get the AIC (Ampere Interrupting Capacity) from the label on the breakers.In the US, it would be a surprise if the required AIC exceeded 10,000A and more likely it would be around 5,000ASuppose the residence is served, along with several others, from a 75 KVA transformer at 240V. Full load current would be 75,000/240 or 312 A. Suppose the transformer has 6% impedance so we divide 312 by 0.06 and the maximum fault current is 5,208 A. Quite close to that 5000 AIC number.The actual fault current will be somewhat less due to available fault current on the primary side which is determined by the impedance of the entire electrical system plus the cable impedance on the secondary side between the transformer and the residence.What the heck is transformer impedance? It is the percentage of input voltage required to get a full secondary current to flow in a short circuit. Short the transformer secondary and run up the input voltage until you get full secondary current and express that voltage as a percentage of nominal primary voltage.6% impedance means just 6% of nominal primary voltage will result in full secondary current. But we don't have 6% voltage so we actually get 1/0.06 or over 16 times nominal output current during a fault.A 6% transformer with a 12,000V primary requires only 720V on the primary to achieve full secondary current.I just grabbed that 6% number out of thin air. Yes, I have used transformers with that rating but I have no idea what values apply to residential systems. It could be somewhat less in which case the fault current would increase.I don't know anything about the numbers (voltages, transformer sizing) outside the US.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 19, 2021, 10:47:32 pm
content: G, is this for a residence, or a commercial or industrial complex? Where in the mains AC distribution were you thinking of needing such a fuse? What is upstream of the location for the fuse (eg. what over-current devices are protecting the feed you are concerned about)?Every fuse situation requires some form of assessing the maximum level of current that the fuse could be exposed to (the prospective current level), and whether a particular fuse has a breaking capacity that exceeds such a maximum, and hence should be safe to install in the feed. That assessment can get detailed, as the specs of upstream devices, and sometime even the length and size of upstream cables come in to play. Sometimes regulations, or an electrical drawing dictate what fault current capability is available at a building entrance or a switchboard, and hence that can be the starting point for detailed assessment of what over-current device ratings are needed inside the building or from feeds connecting to a switchboard.Doh - didn't see the domestic device reference!

author: themadhippy
author_rank: Super Contributor
timestamp: January 19, 2021, 11:08:18 pm
content: As its for a domestic appliance im assuming it will have a uk plug on it,the BS1362 fused fitted in these is rated at 6KA,as are the vast majority of mcb's fitted in domestic consumer units.If you really want to cover your arse the maximum fault current for a 100A single phase supply shouldn't exceed 16KA

author: Northy
author_rank: Regular Contributor
timestamp: January 21, 2021, 10:51:22 am
content: Thanks all for your replies, I understand it now.It is a bit odd how Safety related things seem to be a bit secretive though! I don't know how you are supposed to work out/know some of this stuff.All I can find are fuses rated at 1500A breaking capacity, I'll see if they are sufficient.Thanks again.G