8.5kw shower running on 30A fuse

[zemphis]

Need some help understanding this please.

Out of curiosity I checked my shower installation from the cu side and i think its connected using 6mm cable. The total cable length is approx. 8-10 metres. The shower i have is an 8.5kw running from a 30A rewireable fuse. Its an old MEM unit.

Im confused why the fuse hasnt blown in over 15 years. Would it only blow if there was a fault somewhere? Because the max load on this cable should be about 7.2kw as i understand it? As far as i can tell the cable is partly in trunking and partly loose by the cu and under the bath. Then it is trunked again coming up to the shower. None of the cable is wall buried.

The conclusion i am arriving at is perhaps the fuse is not actually a 30A. Or maybe whoever installed it has somehow put a higher rated one in or bridged two 30A fuses on the board. But apparently 30A is the biggest available. Can anyone clarify any more on this? thanks

 

[rebuke]

a 30A fuse won't blow as soon as you go just over 30A - it would only blow if you either went massively over (e.g. a short circuit or whatever), or you run slightly over it for a long period - given showers are typically 10 minutes or so, it doesn't blow.

(I'm sure someone else will be able to post the relevant graph for a 30A rewireable fuse and show the characteristics - I don't have them to hand)

 

[RF Lighting]

According to the graph, a 30A fuse will allow 50A to flow for over 3 hours before it would operate.

Hence the Cc correction factor

 

[PrinceofDarkness]

A trip down memory lane, from the Thirteenth/Fourteenth Editions' 'Definitions':-

There were two types of excess current protection: Close Protection (MCBs, cartridge fuses etc) and Coarse Protection (semi-enclosed rewireable fuses). These factors still remain essentiallytrue:

Close Protection: Which will operate within 4 hours at 1.5 times the designed load etc.....

Coarse Protection: Which will not operate within 4 hours at 1.5 times the designed load.


So it's hardly surprising that this particular fuse hasn't blown in 15 years......



Lucia.

 

[RF Lighting]

You're showing your age now Lucia

 

[PrinceofDarkness]

Yes indeed, RF.....

I used to lie about my age, but I've now passed that threshold, so I boast about it instead. I really am quite elderly.

However, the fact that someone might cite an antiquarian Regs book, isn't quite an indication of that person's age. He or she might be a 6th Form student of history.

I'm quite interested in the Tudors and Stuarts, but that doesn't mean that I'm 'Victorian'.......


Lucia.

 

[zemphis]

According to the graph, a 30A fuse will allow 50A to flow for over 3 hours before it would operate.

Hence the Cc correction factor

Thanks for the responses.

Thats quite a lot higher than i would expect. I doubt if the shower has ever been on for longer than 30mins.

Does anyone know if it would be worth getting something like this installed:
http://www.toolstation.com/shop/p63252

Or perhaps upgrading the cable from 6mm to 10mm. At the moment i believe the shower cable is below spec and there is no rcd protection. thanks

 

[Taylortwocities]

Your cable is just fine, there's no need to do anything with it
Remember that a fuse is there to protect the cable, and that is what it has been doing, for 15 years.

The addition of an RCD is a great plan but you'd need a competent electrician to do that for you. Certainly the fuse must not be changed from 30amp without stringent tests being undertaken.

Lucia,

Something in here may remind you of your childhood??

Untitled

• Taylortwocities
• 4 Nov 2008
• 4