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Please help a newbie with some basic questions

PostPosted: Mon Apr 30, 2012 2:29 pm
by jonathan

I've done simple PAT testing before but not really appreciated the intricacies of the test settings, just been shown what to connect where and how to push a few buttons until 'PASS' appears on the screen. Having just purchased my own PAT PrimeTest 250 and watched a short training DVD from the vendor I have learned a lot, and already found a problem at my regular work place - a 10A fused mains lead connected to a 3A rated computer. Obviously this is wrong and I want to correct it, but have questions...

Should I be OK with 5A mains leads and fuses on IT equipment that is rated at 3A or less? If the PSU on the computer states 3A, do I have to use a 3A fuse in the plug?

Is it OK to simply replace the 10A fuse with a 3A in the plug for these pieces of equipment (they all use typical IEC computer mains leads), I assume all IEC leads and plugs are rated for 13A fuses but I just wanted to be sure it was a sensible thing to do.

I may already have answered my own question, as I am writing this I am opening up a brand new IEC lead and the plug is rated at 13A, inside is a 5A fuse. Good job I opened this plug too, the cord grip was not screwed down properly. I notice the moulded IEC connector end of this cable actually has 10A 250V stamped on it, does this mean that the 13A plug is not supposed to be on the other end of the flex? The flex itself has nothing on it apart from the place of manufacture. The plug has a label on it "Fitted with BS 1362 5A Fuse", which is what's actually inside it.

Sorry for the basic questions, just want to get it right!

Re: Please help a newbie with some basic questions

PostPosted: Tue May 01, 2012 5:01 pm
by safe + sound
Hi jonathon

European manufacturers should have standardised on 3 amp fuses for appliances up to 700 watt and 13 amp fuses for appliances over 700 watt BUT some low wattage equipment like fridges, computers and monitors have high inrush currents on start-up so manufacturers fit higher rated fuses to avoid spurious operation. Where this is done the plug should be fitted with a label, as you found.

Remember there will be lots of older appliances where wrong sized fuses may have been fitted, labels can get removed and IEC leads don't always stay with the appliance they were supplied with.

It's usual for IT IEC leads to have 10 amp IEC connectors and 5 or 10 amp plugtop fuses.

I suggest you get yourself a copy of the current IEE Code of Practice, 3rd edition and have a read.

Hope this helps

Re: Please help a newbie with some basic questions

PostPosted: Tue May 08, 2012 9:32 am
by jonathan
Thank you. I have now purchased the book and have been perusing it, I find it very easy to follow and reference.

Interestingly, it keeps repeating that the purpose of the fuse in the plug is to protect the mains cable, not the equipment. With that in mind, is the equipment power rating at all relevant for IEC leads? If the IEC lead is rated at 10A, and the plug is rated for and fitted with a 10A fuse, then it surely will do it's job of preventing the IEC lead from heating up or catching fire. Seems to be a bit of a contradiction for IEC leads.

Re: Please help a newbie with some basic questions

PostPosted: Tue May 08, 2012 6:07 pm
by safe + sound
Hi Jonathon
The maximum current carrying capacity of a flexible cable to BS6500 is the amperage it can continuously carry without the cable temperature exceeding 60 degrees C.

Some appliances draw steady current intermittently (fridges, kettles etc) others have varying load (washing machine etc) as it runs through a cycle. I guess bona fide manufacturers have taken this into account when deciding on the size of flex to fit to their product. For example, you will find some jug kettles have 0.75mm2 cable and a 13Amp fuse in the plugtop. Most domestic and IT products don’t have user accessible fuses in the appliance so do rely on the plugtop fuse for protection.

What I’m trying to say is don’t expect real life to comply 100% with the Code of Practice. There are a few grey areas where it contradicts itself. It looks as though a new edition is being prepared so lets hope these issues will be dealt with.

Re: Please help a newbie with some basic questions

PostPosted: Tue May 08, 2012 7:22 pm
by Jim Wallace
Hi Jonathan

It can be confusing when working out fuse ratings. A plug top fuse is in place to protect the mains cable (as stated in the IET CoP) and not the appliance, though there is no harm in reducing the fuse rating to provide additional protection. As safe and sound points out, we are aiming to avoid excessive temperatures and reduce the risk of fire. As an aside, it is the fire risk that is often overlooked when thinking about in-service inspeciton and testing. We tend to focus on reducing the risk of electric shock but faulty appliances and leads are the single biggest cause of fires in the workplace......

The IEC connectors used on PC leads are rated for a maximum continuous current of 10A and so the maximum rating of the lead assembly is 10A (even though the mains plug is 13A rated) so the fuse rating should never exceed 10A. When used with an appliance with a lower current rating you can reduce the plug to fuse rating at this provides additional protection.

I would echo the comments from Safe and Sounds - read through the code of practise and familiarise yourself with the ratings for standard cable sizes and the appropriate fuse rating and use this information/knowledge to carry out a proper visual inspection of mains leads, fuses and plugs. It's surprising how many appliances do not conform from new or the instances where a repair has been carried out using the incorrect cable size.

Good luck

Re: Please help a newbie with some basic questions

PostPosted: Mon May 14, 2012 7:53 pm
by jonathan
Thank you for your help here, very much appreciated.