To Fuse or not to Fuse?

[diyer56]

Hi,

I am wondering if someone can offer some advice please?

I have a 12v power supply which I will be using to provide power to a single light fitting (LED). The power supply was not provided with any instructions and has bare wires ready for connection to the power supply.

I have attached an image of the power supply packaging which provides further information on the device.

I intend to connect the power supply to the lighting circuit. The circuit is protected by a 6 amp MCB. Do I need to add a FCU before the power supply in order to fuse the connection with a 3 amp fuse or can I connect this directly to the switched supply please?

Thanks in advance for your help.

View media item 65168

 

[bernardgreen]

My thoughts are that if ( when ) that unit melts down the 6 amp MCB will allow a lot of power into the failing unit. ( could be 1.5 kWatts )

Putting a smaller ( 1 amp ) fuse in an FCU feeding the unit has benefits....

 

[diyer56]

Ok, thanks Bernard.

I will have to pick up a 1 amp fuse.

 

[stillp]

My thoughts are that if ( when ) that unit melts down the 6 amp MCB will allow a lot of power into the failing unit. ( could be 1.5 kWatts )

Putting a smaller ( 1 amp ) fuse in an FCU feeding the unit has benefits....

You could say the same about anything attached to the lighting circuit, such as any transformer-fed luminaire, fluorescent ballast, etc. I'd wire it direct.

 

[PrenticeBoyofDerry]

I intend to connect the power supply to the lighting circuit. The circuit is protected by a 6 amp MCB. Do I need to add a FCU before the power supply in order to fuse the connection with a 3 amp fuse or can I connect this directly to the switched supply please?

If I have read your question correctly, in order to down fuse a 6A circuit to 3A, then a FCU would be the ideal option. The unit pictured may not need a 3A supply current as 1A would be okay, but I doubt a 3A would harm it.

 

[JohnW2]

My thoughts are that if ( when ) that unit melts down the 6 amp MCB will allow a lot of power into the failing unit. ( could be 1.5 kWatts ) Putting a smaller ( 1 amp ) fuse in an FCU feeding the unit has benefits....

You could say the same about anything attached to the lighting circuit, such as any transformer-fed luminaire, fluorescent ballast, etc. I'd wire it (and everything else on the lighting circuit) direct.

Indeed, I essentially agree with you, and personally would just wire it direct.

In general, I'm sympathetic to Bernard's view that (despite electricians regarding OPDs as being there only to protect cables) there is much to be said for protecting appliances/equipment as much as possible by OPDs (if they don't have any/adequate internal protection). Hence, for example, I would generally use a 1A or 3A fuse in a BS1363 plug if that were adequate for the load, even if a much higher rating of fuse would be adequate to protect the cable alone. However, it makes sense to be consistent, and I see no reason for the OP to 'single out' that one transformer for 'fusing down' if everything else on the lighting citcuit (such as items you mention) are protected only by the 6A MCB. More generally, like yourself, I do not think that the theoretical 'benefit' (per Bernard) would really be adequate to justify the hassle of littering a 6A lighting circuit with 1A or 3A FCUs.

Kind Regards, John

 

[diyer56]

If I have read your question correctly, in order to down fuse a 6A circuit to 3A, then a FCU would be the ideal option. The unit pictured may not need a 3A supply current as 1A would be okay, but I doubt a 3A would harm it.

This is what I meant thank you. Whether to down fuse the transformer to 3 amp (or 1 amp as now suggested by Bernard) using a FCU connected to the switched live supply or just connect the mains supply cable to the switched live supply with a junction box.

You could say the same about anything attached to the lighting circuit, such as any transformer-fed luminaire, fluorescent ballast, etc. I'd wire it (and everything else on the lighting circuit) direct.

You're right, I didn't think of it like this. My living room light fitting has 7 LEDs and has a similar transformer housed within the 'rose'.

As the connection is being made in the attic, and I have an FCU to hand, I have added the additional protection. I only have 3,5 and 13 amp fuses but will buy a 1 amp as recommended by Bernard when I pop out this afternoon.

Thank you all very much for your helpful replies.

 

[JohnW2]

You're right, I didn't think of it like this. My living room light fitting has 7 LEDs and has a similar transformer housed within the 'rose'.
As the connection is being made in the attic, and I have an FCU to hand, I have added the additional protection. I only have 3,5 and 13 amp fuses but will buy a 1 amp as recommended by Bernard when I pop out this afternoon.

It certainly won't do any harm but, as you will have seen, many/most people probably would not bother, particularly when nothing else on the lighting circuit enjoys any such 'additional protection'. If you do install an FCU, it needs to be 'accessible' for maintenance, inspection and testing.

Kind Regards, John

 

[diyer56]

You're right, I didn't think of it like this. My living room light fitting has 7 LEDs and has a similar transformer housed within the 'rose'.
As the connection is being made in the attic, and I have an FCU to hand, I have added the additional protection. I only have 3,5 and 13 amp fuses but will buy a 1 amp as recommended by Bernard when I pop out this afternoon.

It certainly won't do any harm but, as you will have seen, many/most people probably would not bother, particularly when nothing else on the lighting circuit enjoys any such 'additional protection'. If you do install an FCU, it needs to be 'accessible' for maintenance, inspection and testing.

Kind Regards, John

Thanks John, it is within the attic so can be accessed fairly easily. In future I won't lose any sleep if I need to connect one directly.

 

[PrenticeBoyofDerry]

This is what I meant thank you. Whether to down fuse the transformer to 3 amp (or 1 amp as now suggested by Bernard) using a FCU connected to the switched live supply or just connect the mains supply cable to the switched live supply with a junction box.

I don't see any issue by connecting directly lighting circuit without down fusing, but if your were to down fuse logically speaking taking load rating in to consideration a 3A. or 1A will be fine

As the connection is being made in the attic, and I have an FCU to hand, I have added the additional protection.

If the unit is to be fitted there, then make provision for ease of access for maintenance, inspection and testing.

 

[EFLImpudence]

No need.

 

[JohnW2]

No need.

Quite so.

It certainly won't do any harm but, as you will have seen, many/most people probably would not bother, particularly when nothing else on the lighting circuit enjoys any such 'additional protection'.

Kind Regards, John.

 

[bernardgreen]

I have no problem with normal light fittings being connected to a 6 amp circuit as there is very little in them to go wrong. Just wires and the lamp bulb.

It is when the fitting includes a collection of potentially inflamable electronic components and various semi toxic materials that I feel fusing down can reduce the damage if the electronic modules fail.

That said good quality electronic transformers will internally have a fusible track or component on the Live input. This fusible item goes open circuit when the device starts to fail. They also have the necessary clearance between the Live and other components so that when the fusible item goes open circuit there is no way for the Live to jump across to other components beyond the fusible item. For these good quality items fusing down is not necessary.

Some of the low cost items produced in dubious "factories" do not have these fusible items so can literally explode when they fail.

 

[JohnW2]

I have no problem with normal light fittings being connected to a 6 amp circuit as there is very little in them to go wrong. Just wires and the lamp bulb. ... It is when the fitting includes a collection of potentially inflamable electronic components and various semi toxic materials that I feel fusing down can reduce the damage if the electronic modules fail.

Sure, we understand your point, and, as you know, I don't disagree with it conceptually. However, the OP has told us that he has other such transformers on the same circuit which don't have any 'additional protection', so it is questionable as to whether he will gain much by 'singling out' just the new one for such special treatment. More generally, I would suggest that it's probably extremely rare to see any such 'transformers' etc. fed via FCUs - and I think it would have been wrong not to make the OP aware of this, despite your views.

That said good quality electronic transformers will internally have a fusible track or component on the Live input ... Some of the low cost items produced in dubious "factories" do not have these fusible items so can literally explode when they fail.

True. Another point, however, (which is the basis of many of the regulations about protection) is that 'medium impedance faults' (i.e. ones of too high impedance to operate the magnetic trip of an MCB/RCBO) are very rare. The great majority of faults (particularly those which cause equipment to 'literally explode') will usually result in very rapid operation ofthe circuit's protective device.

Kind Regards, John

 

[yorkspark]

Wont fitting a 1a fuse technically be non compliant with bs7671 now as only 3,5 & 13a recognised in the BGB!

But in the real world i would be inclined to connect directly to the led driver and not bother with the fcu