Help please on the construction of layout power supply.

[dasatcopthorne]

Hi Guys.

 

I need to renew a Club power supply but I need some help with fuses please.

 

I intend to use one of the below transformers to power two Kent Panel hand held Heavy Duty controllers which are capable of putting out 3amps. The controllers have short circuit protection built in.

 

The trannies will be in a steel box sited on the floor with a lead taking the low voltage to the layout where wiring will go to DIN plugs for the controllers.  I intend to earth the transformer and the steel box to the mains. We use Common return.

 

My problem is Fuses.

 

What mains fuse should I use, 3 or 5amp?

 

I also want to fuse the transformer ac outputs (just in case) Should these be 3 or 4 amps on each one? My school boy calculations make each output capable of 3.33 amps. Is this correct please.

 

Many thanks in advance.

 

Dave Smith

 

 

[Suzie]

Since the transformer is 100VA a 3A fuse in the mains plug will be more than adequate (I=P/V gives 100VA/(115V+115V)=0.43A. Might need a bit more than 1/2A in case the mains voltage goes above 230V to 265V).

 

You will probably need to look at 5A time delay fuses on the outputs to allow for current surges and such like. In the event of serious sustained overload that is not dealt with by the overload protection in the controller a T5A fuse should blow if the wiring is adequate.

 

Think about using an IEC socket for the mains input. These are available with integrated fuse holders which will allow the fitting of a suitable fuse and avoid the need for worrying about strain relief on the mains cable which can be an issue when attached to a heavy item like a transformer and make separate transportation of the cable a lot easier. Think about using a socket for the AC output too.

 

DIN sockets are not the best for this type of application - think about using something a bit more robust unless you already have a defined standard that uses them. 'D' connectors and XLR connectors are both cheap and readily available and much more robust than DIN connectors, and my personal favourite the Speakon connector might be useful too for connecting the AC outputs to the transformer.

[cliff park]

I guess you realise you need to connect the two inner tags of the input, (primary), and use the two outer tags for your mains. In theory 1amp would be about right for your input fuse, and 5 amp would be ok for output. These should all be mounted such that they can be changed without opening the steel case, and clearly labelled, what they feed and amperage. That way anybody other than you cannot go wrong. The bayonet type holders would be good. Whatever you use make sure they are common and easily available. People do silly things when fuses blow. Make sure that the live end is the furthest away from the outside.

[66C]

Hi Dave

 

For the output side of the transformers have a look at this type of circuit breaker:

 

http://www.maplin.co.uk/c/components/fuses#Category:Circuit%20Breakers&sort=&page=1&productsPerPage=12

 

They are also available from other suppliers.

 

Regards.

[Suzie]

Bayonet type fuse holders are only suitable for low voltage because it is impossible to insert a good fuse without risk of touching a live bit - the IEC sockets with integrated fuse holder make it impossible to touch any live bits when changing the fuse.

[cpman46]

IEC Fused inlet connector with on/off switch and fuseholders fitted to a metal enclosure housing a control panel:

 

 

The fuseholders are of the type where it is impossible to insert the fuse without first inserting them into the removable bayonet cap of the fuseholder.

 

The XLR connector above is used to connect the DCC track voltage to an isolated district via a double pole 15A switch.

 

Regards

Mike C

[dasatcopthorne]

Thank you all for the advice.

 

I have taken it all on-board and will be using the appropriate components for the new power pack.

 

Cheers

 

Dave

[dasatcopthorne]

One more question please guys.

 

I am intending to build 3 power supplies into one project case. This is so that only one unit is carried around.  Nos 1 & 2 are already cased and have output protection. No 3 will be a chassis mount tranny which I shall fit a Bridge rectifier and 3amp auto reset cutout to.

 

Could you please confirm what size mains fuse I need to fit in the single 3 pin plug, working on 240 vac mains.

 

Here's what I think from what I've learnt from earlier posts.

 

No 1. output.  15vac @ 5amps (75va)   = .31

No 2  2 x outputs @  16v dc       (32va)  =  .13

No 3  2 x outputs @ 12v dc        (50va) =  .21

 

Total                                                          .65

 

Is a 3amp mains fuse correct please?

 

Cheers

 

Dave

[Suzie]

3A is good for around 650VA @230V (you are 157VA so you are well within that). Just make sure that your mains lead has at least 0.5mm² conductors and you will be fine.

[Crosland]

Being pedantic, the fuse in the plug top is there only to protect the lead between the plug and the equipment. It should be sized appropriately for that.

 

The equipment should include it's own protection, as appropriate.

 

Andrew

[Brian]

Being pedantic, the fuse in the plug top is there only to protect the lead between the plug and the equipment. It should be sized appropriately for that.

 

The equipment should include it's own protection, as appropriate.

 

Andrew

Surely if that's the case all domestic appliances - vacuum cleaners, electric fires, kettles, fridges etc are not protected, only their mains flex is!

I wonder if your confusing a plug-in style mains cable where the lead plugs into an appliance? The mains lead/cable is then a separate item, and for PAT checking would be tested and recorded as two seperate items - The mains lead (often with a euro C13 socket or figure 8 plug) and the appliance, where the lead is a plug in fit to the appliance. But even then there is no requirement to fit addition fuse or CB protection to the device.

[Grovenor]

Generally the fuse in the plug protects both lead and the conected appliance, the wiring within the appliance is generally comparitively short and a fault occuring there will draw enough current to blow the fuse in the plug. For a transformer the plug fuse is adequate protection for the primary side, you need fuses or circuit breakers, or thermal cutouts on the secondary side as the cabling on that side is likely to be unable to carry the full fault rating of the transformer.

Putting a primary side fuse in the transformer, eg using a fused IEC connector just gives you an additional fuse discrimination problem with the plug fuse for negligible benefit.

Regards

[Crosland]

Surely if that's the case all domestic appliances - vacuum cleaners, electric fires, kettles, fridges etc are not protected, only their mains flex is!

 

A fault in the appliance will cause a higher than normal current to flow in the lead. The plug top fuse must be rated such it blows within the time for which the lead can handle the fault. The appliance may well be destroyed by the fault, it's not the purpose of the fuse to protect the appliance, even if that is sometimes the practical outcome.

 

I wonder if your confusing a plug-in style mains cable where the lead plugs into an appliance? The mains lead/cable is then a separate item, and for PAT checking would be tested and recorded as two seperate items - The mains lead (often with a euro C13 socket or figure 8 plug) and the appliance, where the lead is a plug in fit to the appliance. But even then there is no requirement to fit addition fuse or CB protection to the device.

It's nothing to do with PAT or detachable leads. I didn't say there was a requirement to fit anything to the device, only that it be fitted, as appropriate (perhaps I should have said "if, or as, appropriate").

 

Exactly the same principle applies to the fuses or circuit breakers in your consumer unit. They are there only to protect the fixed wiring in your house.

[dasatcopthorne]

Thank you again for those of you that have helped.

 

Much appreciated.

 

Dave.

[Brian]

The fuse in a UK plug (correctly referred to as a plug top) is ether 3Amp or 13Ampere though other current ratings such as a 5amp fuse can still be found,  it is fitted to protect the appliance and the appliance cable / lead from excessive overload or fault currents.  These can occur between Live and Neutral or Live and Earth.   A common example can be a table lamp, where the filament lamps filament fails usually at switch on and this will in turn rupture the plug top 3Amp fuse before it causes the ring main 32 Amp CB to trip or 30 fuse to rupture.

In a UK domestic situation conventionality a ring main circuit is used to feed all the socket outlets. There may be more than one ring circuit installed, as today it is not uncommon to find three in a domestic installation - Kitchen sockets and any fixed outlets, all downstair socket outlets and a third for all upstairs socket outlets, some may even go further and have more ring mains installed! Older installation may only have the one.  Each of the domestic ring main circuits wiring (Depending upon the age of the installation) will be protected by either a rewirable or cartridge fuse of 30Amp rating of a Miniature Circuit Breaker (MCB) with a current rating of 32Ampere. Some may even employ the use of a trip known as a RCBO but that's another issue. Nether fuse or MCB will normally operate under or at their maximum rated loading. In fact most rewirable fuses will require some 1.8 ish times the rating before rupturing while a CB of the type B rating (As found in most domestic installations) will normally trip at 3 to 5 times its rating but is far faster in its operation than a rewirable fuse can normally rupture. Of course no fuse or CB should be run at its maximum rating as the circuit then needs correcting/altering to allow for the increased loading. The allowance for ambient air temperature and loading needs to be calculated by the installer.   Then we have something called circuit discrimination, whereby the down stream items are all protected by a lower current rated device - Normally the plug top fuse. Upstream domestic installations are protected nominally by a 100Ampere fuse (may differ depending on loading needed) fitted into the meter supply leads. Note1; this fuse is not user replaceable and is the property of the electricity supply company.  Note2; rupturing/trip rates quoted are nominal and may well differ.

 

As an electrical engineer and also now a PAT tester and qualified to the IEE 17th edition, I find some comments made on open forums somewhat bizarre, possibly risky and at times misleading to those who are reading them and who are probably not electrically minded.

 

To be honest and IMO, all this discussion around mains fusing ratings, mains cables etc shouldn't be discussed here.  If anyone isn't sure then the only advice should be... Get a qualified electrician or electrical engineer to undertake the work for you.   Safety of both the individual and the property has to be paramount!