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itolond

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

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?

13th Dec 2017 8:11am

grafty99

Member Since: 15 Aug 2012
Location: North Devon
Posts: 4788

Really you only need one fuse per circuit, so that the relay and load are protected. Additional fuses introduce unnecessary potential for reliability problems

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
Instagram http://www.instagram.com/george_grafton

13th Dec 2017 8:28am

itolond

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

So in essence Assy -switch - relay would be overkill to have a fuse

13th Dec 2017 8:31am

grafty99

Member Since: 15 Aug 2012
Location: North Devon
Posts: 4788

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

13th Dec 2017 9:18am

itolond

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

gotrcha

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

correct

13th Dec 2017 9:41am

grafty99

Member Since: 15 Aug 2012
Location: North Devon
Posts: 4788

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
Instagram http://www.instagram.com/george_grafton

13th Dec 2017 11:04am

itolond

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

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?

13th Dec 2017 11:14am

Martin
Site Admin

Member Since: 02 Apr 2007
Location: Hook Norton
Posts: 6613

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

13th Dec 2017 11:36am

itolond

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

GOT IT

13th Dec 2017 1:55pm

custom90

Member Since: 21 Jan 2010
Location: South West, England.
Posts: 20504

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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13th Dec 2017 3:32pm

itolond

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

ok i get it

appreciated

15th Dec 2017 2:42am

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