Page 11 - Fuse link Catalogue

DISCRIMINATION BETWEEN FUSE LINKS

Discrimination Between Fuse Links In service, the short circuit fault conditions encountered are
usually less exacting than those produced in proving tests on fuse links and associated equipment.
AS2005.10-1988 & BS88 Part 1:1988 states that fuse links experience fault currents which produce
pre-arcing times longer than 0.01 second in most cases, and on that basis fuse links complying
with the standard are deemed to discriminate with each other when the ratio between the current Figure 1
ratings of ‘major’ and ‘minor’ fuse links is 1.6:1 (see Figure 1).
Whilst the AS2005 & BS88 statement is reasonable in relation to 240V applications fault currents in
major installations can be much greater.
However, even in the latter cases conditions are less onerous than those encountered in test stations
(in particular, the circuits are usually three phase with relatively high power factors).
2
In practice therefore, the I t values of type ‘T’ fuse links are significantly less the ones listed on
pages 12-15 and they will discriminate with each other at fault levels up to 80kA, 415V, if the
relationship between ‘major’ and ‘minor’ ratings is as given in the table. In most cases the
discrimination ratio is 1.6:1, or less, and this provides economic benefits in modern installations.
Tests have been taken to prove this level of performance.
The above table also gives details of combinations which will discriminate at 415V, 550V and 660V.
Effect of High Enclosure Temperatures
In accordance with AS2005.10-1988 & BS88:Part1:1988 type ‘T’ fuse links are suitable for use in
O
ambient air temperatures (Ta) not exceeding 40 C with a mean value measured over 24 hours of
O
not more than 35 C.
When fuse links are fitted in enclosures, it is the temperature within the enclosure (Te)* which
determines whether it is necessary to derate fuse links. No derating is needed in following cases:
’Minor‘ Minimum rating (Amp) of ‘Major’
fuse link fuse link that will discriminate with Nominal Maximum load current
rating the ‘minor‘ fuse link at the voltage Fuse at these fluid environment
shown at prospective currents
Rating temperatures (Te)*
up to 80kA
Amp 55° 60° 65° 70° 75°
Amp 415v 550v 660v 80 75 70
800 1250 – – 100 95 90
750 1250 1250 –
125 120 110
710 1250 1250 1250
160 145 135
670 1250 1250 1250
630 1000 1250 1250 200 190 180 170
560 800 800 1000 250 235 225 210
500 750 800 1000
315 300 285 270 255
450 670 750 800
355 350 330 315 295
400 630 670 750
355 630 630 750 400 400 380 360 340
315 500 500 630 450 425 405 380 360
250 400 450 500
500 475 450 425 400 380
200 315 400 400
560 540 520 495 465 440
160 250 315 315
125 200 200 250 630 600 570 540 510 480
100 160 200 200 670 650 615 585 550 520
80 125 160 160
710 700 665 630 595 560
63 100 160 160
750 750 710 670 630 590
50 80 100 125
40 63 63 100 800 760 720 680 640 600
35 50 63 80 1000 950 900 850 800 750
32 40 50 63
1250 1140 1070 1020 960 900
25 40 40 40
* Fluid environment temperature (Te) is the temperature inside
20 32 32 35
16 25 25 32 the enclosure containing the fuse link.
IPD INDUSTRIAL PRODUCTS Phone: 1300 556 601 www.ipdgroup.com.au
9