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itolond



Member Since: 16 Jan 2016
Location: Singapore
Posts: 193

Singapore 1997 Defender 110 300 Tdi SW Alpine White
Silly question - fuses and relays
Hi Forum,

i understand a fuse is located between load and power (typical setup)

in the case where you have a relay box with incorporated fuses is normal to have a fuse on the power side (to relay) as well as on the relay to load item.
PS the main 12V power also has a breaker switch to battery

i am guessing so as if there is a short on the switch side (eg SPST from accy to relay?)

whats the go?
Post #670638 13th Dec 2017 8:11am
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grafty99



Member Since: 15 Aug 2012
Location: North Devon
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United Kingdom 2002 Defender 90 Td5 HT Caledonian Blue
Really you only need one fuse per circuit, so that the relay and load are protected. Additional fuses introduce unnecessary potential for reliability problems Thumbs Up 2002 90 Td5 Station Wagon
1990 Vogue SE
Triumph Tiger Explorer 1200
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Post #670639 13th Dec 2017 8:28am
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itolond



Member Since: 16 Jan 2016
Location: Singapore
Posts: 193

Singapore 1997 Defender 110 300 Tdi SW Alpine White
So in essence Assy -switch - relay would be overkill to have a fuse
Post #670640 13th Dec 2017 8:31am
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grafty99



Member Since: 15 Aug 2012
Location: North Devon
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United Kingdom 2002 Defender 90 Td5 HT Caledonian Blue
Not sure I understand what you're saying there. What do you mean by assy?

Any circuit needs a fuse, but only one fuse per circuit. Not a fuse each side of the relay Thumbs Up 2002 90 Td5 Station Wagon
1990 Vogue SE
Triumph Tiger Explorer 1200
Td5 90 Thread http://www.defender2.net/forum/topic50767.html
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Post #670646 13th Dec 2017 9:18am
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itolond



Member Since: 16 Jan 2016
Location: Singapore
Posts: 193

Singapore 1997 Defender 110 300 Tdi SW Alpine White
gotrcha

Accessories live -> Switch -> relay -> fuse -> load

correct
Post #670650 13th Dec 2017 9:41am
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grafty99



Member Since: 15 Aug 2012
Location: North Devon
Posts: 4784

United Kingdom 2002 Defender 90 Td5 HT Caledonian Blue
It should be

Accessory live --> fuse --> switch --> relay --> load

The fuse protects the whole circuit not just the load 2002 90 Td5 Station Wagon
1990 Vogue SE
Triumph Tiger Explorer 1200
Td5 90 Thread http://www.defender2.net/forum/topic50767.html
Tdi 110 Thread https://www.defender2.net/forum/topic69562.html
RRC Thread http://www.defender2.net/forum/topic54492.html
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Post #670662 13th Dec 2017 11:04am
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itolond



Member Since: 16 Jan 2016
Location: Singapore
Posts: 193

Singapore 1997 Defender 110 300 Tdi SW Alpine White
Ok?

So the fuse only protects the switch circuit. The relay live to load is in itself another circuit albeit activated via a electro mechanical switch (relay)

So in this case the fuse is only protecting the Low amp circuit?
Post #670665 13th Dec 2017 11:14am
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Martin
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Member Since: 02 Apr 2007
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Posts: 6460

United Kingdom 2015 Defender 90 Puma 2.2 XS CSW Montalcino Red
You have two circuits, the circuit which controls the relay (switch for the relay coil). Normally the supply for this would be tapped from another circuit as it is very low current.

Then there is another circuit, the one switched by the relay. Generally you will take a new supply for this, and place a fuse before the relay.  1988 90 Td5 NAS soft top
2015 D90 XS SW
Post #670667 13th Dec 2017 11:36am
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itolond



Member Since: 16 Jan 2016
Location: Singapore
Posts: 193

Singapore 1997 Defender 110 300 Tdi SW Alpine White
GOT IT
Post #670690 13th Dec 2017 1:55pm
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custom90



Member Since: 21 Jan 2010
Location: South West, England.
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United Kingdom 
You will sometimes have for distribution boxes or Relay boxes etc have double fusing.

However, as Martin states all of that is correct.

But you will often get, a master supply with larger cable i will use an example:

100A cable > fused at 80A with in a max of 20" from battery post ideally closer.
Supplying a live bussbar or similar ditribution, down to either two loads say 40A fused each. > to any purpose required.

Or to a Relay box for example with say 4x Relays with 20A capacity each with in-built standard fuses.

Any issues with the circuit past the Relay the 20A fuse will blow, other circuits Remain live and active as well as the master supply.
Any issues with the master supply cable run, the 80A master fuse will blow but all circuits will be down.

I personally fuse in-line nearest supply source and use non fused Relays as they aren't always required.
It varies by fit location, for most uses I don't use a fused Relay but in distribution blocks and boards they can be useful.

Fusing is to protect the cable from fire, and not a device so the cable must be rated higher than the fuse rating.
And both should be rated above the load, fuse should be below cable current capacity but around 5 - 10A above device current load.

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Post #670714 13th Dec 2017 3:32pm
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itolond



Member Since: 16 Jan 2016
Location: Singapore
Posts: 193

Singapore 1997 Defender 110 300 Tdi SW Alpine White
ok i get it

appreciated Very Happy
Post #670991 15th Dec 2017 2:42am
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