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MG MGB Technical - Secondary Fuse Box – Fused Supply?



I am fitting a secondary fuse box solely to run a Brantz Timer and tripmeter and they require connecting both live and earth direct to the battery.

They both require a 2A fuse. Would it make sense to put a 5A fuse on the live feed to the fuse box, or is that being too cautious, there are plenty of un-fused feeds elsewhere?

It's just 2 more connections to potentially fail if a fuse is not required.

Mike Dixon

Mike,
it doesn't make sense to me 2 x 2 = 4 not 5, to be simplistic. Yes I think you're being too cautious.

And the more connections you put into a circuit the more potential for possible problems. I'd have each fuse as close as possible to the unit so offering as much protection to the unit as possible.
Nigel Atkins

Sorry I was rushed away for tea, I meant to put I assume (always dangerous) as I know nuffink about such equipment, that as they want direct supply to the battery the manufacturer wants/needs you to have a clean supply so another reason to keep connections to a minimum.

Are you given any more installation info or specs as if you have the two 6V batteries they're a long way back with extra cable connections.
Nigel Atkins

Fitting a fuse in the power supply depends mostly on where you are getting that power supply from
If it's a short wire that has no chance of touching anything and rubbing through you don't need a fuse but if it's a longer wire and running right back direct to the battery or similar then yes, a good idea to have a fuse at the battery (supply) end of the wire

willy
William Revit

If 'direct from the battery' is specifically stated in the instructions that may be significant for electronic equipment, there can be large voltage dips and spikes on cars like ours as things are turned on and off. I've had to design circuits to run off half-voltage which allows me to include suppression to ignore them.

Having said that if the equipment is that sensitive then restarting after a stall with the equipment running is probably going to upset it.

Having a cut-off switch without a bypass (to stop the alarm draining the battery) I've run a supply for my clock right back to an in-line fuse on the 12v battery connector - as said that fuse MUST be right at the battery to protect against shorts if the wiring gets damaged.

If 'direct from the battery' is not a specific requirement then an in-line off the brown at the fusebox will give the required protection against shorts, but maybe not against spikes and dips.

A 'cleaner' supply can be had from the battery cable terminal on the starter solenoid, but again will need the in-line to be right on that terminal.

The earth is less of an issue assuming the battery connection to the body is clean, you can pick that up from a clean body connection near the equipment.
paulh4

The cable is getting a clean supply direct from the single 12v battery to the Brantzs on the centre console/glove box.

I agree it would be best nearer the battery, but in terms of accessibility I wonder if an inline one behind the centre console might be better?

The alternative is a panel mounted single
inline blade, with an LED fuse on the heelboard next to where I have my master switch. It will need protecting though


Mike




Mike Dixon

Mike
A fuse up at the centre console is basically useless as a protection for the supply --It's at the wrong end of the wire---it will do nothing and you already have the two 2A fuses there anyway
IF you're going to fuse the supply wire it needs to be at the power source end of the wire

I'd be tempted to just run your wire through some spaghetti or use some double insulated marine wire, up to your 2 x 2A fuses and take the punt
If it's securely mounted out of harms way it should be ok

The instruction to run the wires back to the battery is probably to stop people hooking them up to the alternator or ignition coil or somewhere else that might have electrical noise in the circuit--Running your wire straight off your isolator switch or off the starter solenoid would be fine

willy
William Revit

Exactly so.

"The cable is getting a clean supply direct from the single 12v battery to the Brantzs on the centre console/glove box."

If this means you have already run a wire in direct from the battery, then you must put a fuse right at the battery as previously said. If you don't do that and your wire shorts to earth then it will likely burn the car up.
paulh4

Mike,
looking at Brantz's website and a sample installation instructions there's very little info or any reasoning for their instructions, this might be deliberate. There must be very many Bs and other similar classics running these units so I wonder if it's a case of Brantz being cautious from past experience of problems from customers, including 'noisy' wiring as Willy has put.

I don't know over much about electrics but I do know from past experience of expensive electronics (hi-fi) that some units wants power drawn from a dedicated individual supply from the power origin. That'd be positive and negative from the 12v battery posts directly to the units as you have it.

Whether this is OTT I don't know and the 2A fuse(s) seem odd as they're not supplied already fitted to the tails of the units. 2A matches what's often used for classic radio installation, is that a coincidence or convenience or the units actual need 2A. I'm surprised the units don't have an accessible onboard fuse if it's that important but again that might cause problems for and with customers use.

As I see it the fuses should be units end and its as Willy has already put for adding in another supply fuse. Adding in anything other than the two 2A fuses requested strays from the 'clean' supply so you might as well have went a bit further with the other suggested routes.

Paul and Willy know what they're talking about whereas I hardly do so I was assured to see that they were as surprised as me at Brantz (unexplained) wiring requirement.
Nigel Atkins

Paul,
unless I'm missing something, which often happens, just the way I see it on the little info available, unless the wire(s) is really thin stuff it's probably 8 or 17 type so the 2a fuse would melt first. There's already unfused cables carrying a lot more fire potential in them on the car already to worry about more than these supplies.

All fuses on the battery post makes sense to me but that's not how these cars are particularly the B (and ones with two batteries joined by a cable) and there's cables to solenoid, starter, headlights.

If the 2a fuse is there as there's nothing stated about the wires to supply the unit then they're there as (partial placebo?) protection for the unit.
Nigel Atkins

If the fuse is at the unit then as I and Willy have pointed out any wire between the battery and the fuse is unprotected. If that shorts out it will burn. A fuse only protects wiring and equipment BEYOND the fuse, not back towards the supply.

If the seemingly short length of wire between the fuse and the unit were to short, or the the unit itself, then yes the fuse would blow. But not otherwise.

Electronic equipment usually has fusing of a lower rating then the standard ones on the MGB, but that is suit the requirements of the unit, not the owner-provided wiring bringing the power to it. The North American Sequential Seat-belt System on the MGB had it's own 500mA fuse, so in the event of an internal fault that would blow rather than smoking the unit.

Modern cars often have two or three fuses cascading in series, of progressively lower ratings, feeding progressively thinner wiring, depending on the load of what lays beyond. Usually every single component and all the wiring bar the starter motor has a fuse or fusible link protecting it.

As we know the wiring on the MGB has very little by way of protection.
paulh4

The attached may help. Consider a short-circuit to earth at 'A', and one at 'B' (hopefully not at the same time ...). At 'A' current will flow from the battery to earth - a short-circuit - before it reaches the fuse, so the fuse is bypassed and it will not blow. In this case a very high current will flow and the wiring will burn. But with a short-circuit at 'B' current is flowing though the fuse and that will blow breaking the circuit and protecting the wiring.

The closer the fuse is to the supply the more wiring it will protect.



paulh4

Sorry on re-reading my post I didn't make it clear I was thinking of heat overload rather than an outright cut short that's why I put all the fuses would be best on the battery post.

The way I saw it was the 2a fuse is being asked for as protection to the unit rather than the wire - but perhaps they do mean 2a at battery.

Looking at another of their manuals it has -
"Connect straight to the vehicles battery posts via a 2 Amp fuse (Not Provided - Available from Brantz) on the live wire, ..."
(but I can't find a 2 amp fuse on their website).

Hands up - I was wrong. It looks like the 2 amp fuse is battery not unit end by the fact their plug kit is unfused.

They ask for direct battery supply yet sell an AUX Plug, with 2amp fitted fuse, LED power indicator and solder on connections - what am I missing this time!



Nigel Atkins

Maybe I've misunderstood. If the current is high enough to damage the wiring but not blow the fuse then it doesn't matter where the fuse is. That's why wiring should always be chosen to suit the normal current drawn by the unit and the fuse to suit the wiring i.e. to blow on a gradually increasing current before the wiring starts to get damaged.
paulh4

Whilst I remained wrong with my idea of fuse location for Mike's installation, looking at the Brantz site and manuals a bit more I think I was right with Brantz being cautious from past experience of problems from customers.

Some of the Manuals have a revision code, others don't and of course the revision could be for other changes including or excluding the bit about connection to battery posts. Plus some instructions just ask for -
" ... (power) cable connects via a 2 amp fuse to a permanent +12 volt supply (i.e. not controlled by the ignition switch etc)". - https://9c954326-9eb0-4214-988c-937c82677015.filesusr.com/ugd/f2f644_5715b237713b470f9290ee2e83de1ab3.pdf

The attached pdf shows where the instruction to wire from battery has been added in.
https://9c954326-9eb0-4214-988c-937c82677015.filesusr.com/ugd/f2f644_1cf245ad69984499b00442cdae56e810.pdf

I might be wrong again but their plug kit wires don't look to be special or have additional shielding.



Nigel Atkins

Opening my eyes more and using the scroll wheel on my mouse I found the Brantz Fuse Kit (BRFUSE) and you get the choice of in-line glass or blade fuse.

https://www.brantz.co.uk/product-page/fuse-kit-brfuse






Nigel Atkins

The instruction in the second links is emphasised by being boxed, with DIRECTLY in capitals, which is pretty conclusive to me that it does have to be connected that way to work properly, and hence needs a fuse right at the battery. Probably for the reasons I mentioned way back.

The cable wouldn't need to be 'shielded' as such as it's not so much high-frequency interference that might be a problem with the sensor cable, just voltage supply fluctuations as other things on the car are turned on and off.

After-market LEDs on these cars flicker as things are turned on and off for that reason and can cause the car to fail its MOT from being 'adversely affected by the operation of other lamps' aka 'discoing'. I did some tests using an oscilloscope and even the fuel pump (on the unfused ignition circuit) caused the fused ignition circuit voltage to momentarily drop by about half as the pump energised, with a much smaller upward tick as it released. 'A' is the zero voltage line, the upper line is 12v, with the pump energising at 'B' and releasing at 'C'.

It also shows a small reduction in voltage while the pump is energised. If the electrics on our cars were 'perfect' then it wouldn't be a problem, but they were never intended to power electronic system when new let alone after 50 years. OEM system include suitable smoothing and suppression to ignore it.



paulh4

As I have 2 Brantz devices fitted ( Timer and Trip). I had a chat with Brantz and went for a blade fuse box on the passenger bulkhead with an earth post next to it. I previously had them with separate inline, in the cabin, 2A fuses. The requirement to be directly fed is for any vehicle.

One twin core wire, about to be fused at the battery (!) , feeds the fuse box/earth post and the 2 Brantz are connected to the fuse box with 2 amp fuses. They are easy to get at and illuminate should they blow.

It is a lot tidier, more secure and easier to access than inline fuses tucked behind the centre console with the associated extra cable to allow me to get at them.

I have not tried this setup on the road yet, for obvious reasons but hopefully they will be free from interference, I see no reason why not.


Thanks you all for you help - I am just looking forward to using them now....
Mike Dixon

Well Done Mike you done the spot on thing, talked with the manufacturers.

My recent point was that the requirement for direct connection to the battery posts has been added in at some time based on the accessories they sell and the Manuals and clear alteration to the one I put up as an attached pdf.

In the body of the text it has -
"Brown wire from the Black (power) cable connects via a 2 amp fuse to a permanent +12 volt supply (i.e. not controlled by the ignition switch etc)"

but then has added further down the page boxed
"Take power DIRECTLY from the Battery Terminals".

I think these plus what Brantz have said gives interpretation to exactly what is really meant by direct but that could be to reign in some customers interpretation of correct wiring.
Nigel Atkins

The supply on my car is very noisy, probably not important back in the day but I had to provide a separate filtered clean supply for the sat nav and dash cam.
You have had excellent advice about the need for the fuse and where it should go. Close to the battery where it will prevent a fire if the wire shorts to ground.
I have fitted fuses elsewhere as well such as white wire overdrive circuit as I think the factory approach of leaving large areas without fuses doesn’t work for me.
Stan Best

I have the overdrive and the fuel pump fused on both my cars. Both came to me having had shorts at the pump judging by home much wiring showed damage, and several I have worked on.

The most convenient place for a fuel pump fuse is where the rear and main harness join, overdrive similar or on the back of the dash manual switch which avoids cutting into wiring. Details vary according to year and market, but if you have a 77 and later with the manual switch on the gear knob it's more important to fuse those as the wiring up the lever has been known to chafe and short out.
paulh4

Those fuses are the next job - 5A for the overdrive and 10A for the pump?

Mike Dixon

I just used standard glass fuses in in-line connectors as there are spares in the fusebox. They don't need to be that high, but there to protect the wiring they don't need to be lower.
paulh4

This thread was discussed between 18/02/2021 and 27/02/2021

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