I have a 8.5kw electric shower which has stopped working, looking in the
fuse box the Wylex (push in type, not mcb) carrier and fuse are burnt!

The shower was fitted many years ago by Southern Electric, so I can only
assume the cable is correct. I replaced the actual shower unit with a new
one, pratically identical one from Wickes/Triton.

Should I be concerned about this?. I can easily replace the fuse and
hopefully catridge, but it has scortched some of the bit the fuse plugs
into - can this be replaced.

any ideas would be appreciated
gna

I have a 8.5kw electric shower which has stopped working, looking in the
fuse box the Wylex (push in type, not mcb) carrier and fuse are burnt!

The shower was fitted many years ago by Southern Electric, so I can only
assume the cable is correct. I replaced the actual shower unit with a new
one, pratically identical one from Wickes/Triton.

Should I be concerned about this?. I can easily replace the fuse and
hopefully catridge, but it has scortched some of the bit the fuse plugs
into - can this be replaced.

any ideas would be appreciated

By my calculations, 8.5KW @ 240v is 35.4A

Maybe somthing to do with it?

Sparks...

"gna03633" wrote in message
7...
I have a 8.5kw electric shower which has stopped working, looking in the
fuse box the Wylex (push in type, not mcb) carrier and fuse are burnt!

The shower was fitted many years ago by Southern Electric, so I can only
assume the cable is correct. I replaced the actual shower unit with a new
one, pratically identical one from Wickes/Triton.

Should I be concerned about this?. I can easily replace the fuse and
hopefully catridge, but it has scortched some of the bit the fuse plugs
into - can this be replaced.

any ideas would be appreciated
gna

8.5 kw shower will use approx 35 amps, check the cable if 6mm or above would
be ok to just upgrade the fuse to 40 amp.

If less then you will need to run in new cable to the shower, preferable
10mm to allow for future upgrades.

Your previous shower must have been less than 7.6 Kw

Dave Jones

Just found a table of fuse and cable ratings for showers here

http://www.diydoctor.org.uk/projects/electricshower.htm

Sparks...

gna03633 wrote:

I have a 8.5kw electric shower which has stopped working, looking in the
fuse box the Wylex (push in type, not mcb) carrier and fuse are burnt!

The shower was fitted many years ago by Southern Electric, so I can only
assume the cable is correct.

I'd assume no such thing...

Should I be concerned about this?

Yes. Such overheating should not occur and if not attended to could
become a fire risk. Possible causes:

- loose terminal screw on live wire coming out of top of fuseway,
- loose screw within fuse carrier (if re-wireable type),
- circuit wired with grossly undersized cable,
- loose fuse contact spring in consumer unit,
- 30 A (red) fuse installed (8.5 kW shower needs a 45 A (green) fuse).

I can easily replace the fuse and hopefully catridge, but it has
scortched some of the bit the fuse plugs into - can this be replaced.

Yes, the "scorched bit that the fuse plugs into" is just a plastic
shroud which (a) keeps fingers away from the live busbar when the fuse
is out and (b) prevents the casual (ab)user plugging 30 A fuses into 5 A
circuits (notice how the width of the slots is graded). Undo the screw
in the centre and it will just pull out - this exposes the live busbar,
so turn off the power at the big switch first, unless you really know
what you're doing. If you go to an electrical wholesaler and buy a new
Wylex fuse assembly this shroud comes as part of it.

Having checked the terminal screws, offer the the new fuse carrier up to
the fuseholder without the shroud in place first (power off) and make
sure that the contacts are tight. The previous overheating may have
annealed the contact springs to the point where the fuse is a loose fit.
If this is the case you may have no alternative but to replace the
whole consumer unit (in which case consider upgrading to a modern split
load MCB unit). If you fit a new fuse assembly into loose contacts it
will just burn again...

HTH
--
Andy

Dave Jones wrote:

8.5 kw shower will use approx 35 amps, check the cable if 6mm or above would
be ok to just upgrade the fuse to 40 amp.

Really? 6mm is ok for 40A? I thought this was only the case for surface
clipped and surface conduit, no?

--
Grunff

I replaced the actual shower unit with a new one, pratically identical one
from
Wickes/Triton.

How sure are you that the "identical" replacement had the same power rating
as the previous one? I strongly suspect your replacement is more powerful,
as a fused one is likely to be at 30A, giving a maximum of 7.2kW. Of course,
it is possible to get bigger fuses, but they aren't that common. Don't go
fitting a bigger fuse unless the cable is found to be OK for the larger
current.

Should I be concerned about this?. I can easily replace the fuse and
hopefully catridge, but it has scortched some of the bit the fuse plugs
into - can this be replaced.

Yes, but fuse boxes always seem to do this. After a few blows, they look
like melted Mozzarella on a pizza. Consider replacing your consumer unit
with a modern one, with RCD and MCB protection.

Christian.

Andy Wade wrote in message ...
gna03633 wrote:

I have a 8.5kw electric shower which has stopped working, looking in the
fuse box the Wylex (push in type, not mcb) carrier and fuse are burnt!

The shower was fitted many years ago by Southern Electric, so I can only
assume the cable is correct.

I'd assume no such thing...

Should I be concerned about this?

Yes. Such overheating should not occur and if not attended to could
become a fire risk. Possible causes:

- loose terminal screw on live wire coming out of top of fuseway,
- loose screw within fuse carrier (if re-wireable type),
- circuit wired with grossly undersized cable,
- loose fuse contact spring in consumer unit,
- 30 A (red) fuse installed (8.5 kW shower needs a 45 A (green) fuse).

I can easily replace the fuse and hopefully catridge, but it has
scortched some of the bit the fuse plugs into - can this be replaced.

Yes, the "scorched bit that the fuse plugs into" is just a plastic
shroud which (a) keeps fingers away from the live busbar when the fuse
is out and (b) prevents the casual (ab)user plugging 30 A fuses into 5 A
circuits (notice how the width of the slots is graded). Undo the screw
in the centre and it will just pull out - this exposes the live busbar,
so turn off the power at the big switch first, unless you really know
what you're doing. If you go to an electrical wholesaler and buy a new
Wylex fuse assembly this shroud comes as part of it.

Having checked the terminal screws, offer the the new fuse carrier up to
the fuseholder without the shroud in place first (power off) and make
sure that the contacts are tight. The previous overheating may have
annealed the contact springs to the point where the fuse is a loose fit.
If this is the case you may have no alternative but to replace the
whole consumer unit (in which case consider upgrading to a modern split
load MCB unit). If you fit a new fuse assembly into loose contacts it
will just burn again...

HTH


I'm concerned by most of the advice in this thread - not just the
above. There coule be a live to earth fault and no bathroom bonding
for all we know. Following any of the advice here could kill you.

You need to find out what the fault is, not start guessing. An
electric shower is not something to muck about with.


NT

N. Thornton wrote:

I'm concerned by most of the advice in this thread - not just the
above. There coule be a live to earth fault and no bathroom bonding
for all we know.

How do you deduce that from the symptoms described?

--
Andy

Firstly thanks for the advice/comments

How sure are you that the "identical" replacement had the same power
rating as the previous one? I strongly suspect your replacement is
more powerful, as a fused one is likely to be at 30A, giving a maximum
of 7.2kW. Of course, it is possible to get bigger fuses, but they
aren't that common. Don't go fitting a bigger fuse unless the cable is
found to be OK for the larger current.

I had the Aquatronic 8500 for a couple of years, it had blown the fuse
once before and was cutting out all the time. I had the manual left from
the previous owner and checked inside the housing for the rating. I
wanted somthing I could replace myself, so bought the Aquatronic 2 from
Wickes - for £60 or so with 2 years warranty. Everything was in the same
place i.e water/cables etc so was a easy swap.

I was concious of the cable and looked along it length I can see (in the
loft) any size mention, nothing written so measuring the size looked OK.


Should I be concerned about this?. I can easily replace the fuse and
hopefully catridge, but it has scortched some of the bit the fuse
plugs into - can this be replaced.

I went to my local electrical place an bought a 40amp Wylex Catridge (in
the manual for the shower it says use a 45A an 6mm. I also got a
'cutting' of 6mm cable so I could compare. Ive taken off the shower cover
and unscrewed the cable and measured it against, what I got given. They
both look the same size, with multiple strands for the core and measure
around 5mm with my ruler.

So I pretty confident the cable is good and the correct size.


Yes, but fuse boxes always seem to do this. After a few blows, they
look like melted Mozzarella on a pizza. Consider replacing your
consumer unit with a modern one, with RCD and MCB protection.

I seem to remember that the fuse did seem quite loose and maybe that
didn't help.

Christian.

"Grunff" wrote in message
...
Dave Jones wrote:

8.5 kw shower will use approx 35 amps, check the cable if 6mm or above
would be ok to just upgrade the fuse to 40 amp.

Really? 6mm is ok for 40A? I thought this was only the case for surface
clipped and surface conduit, no?

--
Grunff

Taken from On Site Guide, Table 6D1 Method 3 (enclosed in conduit on a wall
in trunking etc.) 6mm 2 cables single phase, current capacity = 41 amps

Method 4 (enclosed in thermally insulating wall etc) 34 Amps

Method 1 (clipped) 47 Amps


Most shower circuit's I've seen go straight up the wall across the loft and
down to the shower, so some where in between Method 3 and 4, and at most
going to be using 37 amps for max 30 min at a time.

So a 40 amp fuse will protect the circuit from any occurring faults.

Dave Jones wrote:

Taken from On Site Guide, Table 6D1 Method 3 (enclosed in conduit on a wall
in trunking etc.) 6mm 2 cables single phase, current capacity = 41 amps

Method 4 (enclosed in thermally insulating wall etc) 34 Amps

Method 1 (clipped) 47 Amps


I was reading from the same hymn sheet :-)


Most shower circuit's I've seen go straight up the wall across the loft and
down to the shower, so some where in between Method 3 and 4, and at most
going to be using 37 amps for max 30 min at a time.

But is it safe to make that assumption? Given that we don't know that
there isn't a 5m run of cable running through loft insulation, is it
safe to assume? I'm not being argumentative - I'm geniuinely curious
about the correct way of deciding this.

--
Grunff

I was reading from the same hymn sheet :-)


Most shower circuit's I've seen go straight up the wall across the
loft and down to the shower, so some where in between Method 3 and 4,
and at most going to be using 37 amps for max 30 min at a time.

But is it safe to make that assumption? Given that we don't know that
there isn't a 5m run of cable running through loft insulation, is it
safe to assume? I'm not being argumentative - I'm geniuinely curious
about the correct way of deciding this.

It says similar in the Triton installation for the shower I have:
6mm2
Installed in an insulated wall 32A
in conduit or trunking 38A
Clipped direct or buried in a non insulated wall 46a

Mine goes from the meter along some trunking form 3meters up the non
insulated wall, into the loft above then down the wall then into the
shower.

The biggest problem is decided how big or small he cable is!

Grunff wrote:

Dave Jones wrote:

Taken from On Site Guide, Table 6D1 Method 3 (enclosed in conduit on a
wall in trunking etc.) 6mm 2 cables single phase, current capacity =
41 amps

Method 4 (enclosed in thermally insulating wall etc) 34 Amps

Method 1 (clipped) 47 Amps

I was reading from the same hymn sheet :-)

Wrong hymn sheet, 6D1 (4D1A in the BS) is for singles, not twin & earth.
For T&E look at Table 4D5A (6F in the OSG). For flat cable in
insulated walls you choose from Method 6 (enclosed in conduit in
insulated wall) - 32 amps - or Method 15 (installed directly in an
insulated wall) - 35 amps. Method 1 (clipped direct) is the same rating
though - 47 A.

But is it safe to make that assumption? Given that we don't know that
there isn't a 5m run of cable running through loft insulation, is it
safe to assume? I'm not being argumentative - I'm geniuinely curious
about the correct way of deciding this.

Go into the loft and have a look. If the cable's surrounded in
insulation the effective rating will be 20-something amps and trouble looms.

--
Andy

Andy Wade wrote:


Wrong hymn sheet, 6D1 (4D1A in the BS) is for singles, not twin & earth.
For T&E look at Table 4D5A (6F in the OSG). For flat cable in
insulated walls you choose from Method 6 (enclosed in conduit in
insulated wall) - 32 amps - or Method 15 (installed directly in an
insulated wall) - 35 amps. Method 1 (clipped direct) is the same rating
though - 47 A.

Ok, good point. Still finding my way around the on-site guide. Whoever
put that document together could really do with some instruction on data
structuring.


But is it safe to make that assumption? Given that we don't know that
there isn't a 5m run of cable running through loft insulation, is it
safe to assume? I'm not being argumentative - I'm geniuinely curious
about the correct way of deciding this.


Go into the loft and have a look. If the cable's surrounded in
insulation the effective rating will be 20-something amps and trouble
looms.

Not my loft, I was just joining in :-)


--
Grunff

Grunff wrote:

Ok, good point. Still finding my way around the on-site guide. Whoever
put that document together could really do with some instruction on data
structuring.

Yes it's abysmal, and poorly indexed too. And _still_ contains a
reference to a shower cubical (/sic./) [3.6.1(v)] which appeared in the
previous (blue) edition. It follows the formal structure of BS 7671
much too closely and hasn't been structured with the users' needs in
mind at all. And the division of material between the body and the
appendices is bizarre in the extreme.

Not my loft, I was just joining in :-)

I wasn't really sure what you were getting at there and just joined in
too ;-)

--
Andy

Andy Wade wrote:

I wasn't really sure what you were getting at there and just joined in
too ;-)

I was really just responding to Dave Jones, who said:
"8.5 kw shower will use approx 35 amps, check the cable if 6mm or above
would be ok to just upgrade the fuse to 40 amp."

I was asking whether it would really be ok to fuse the circuit at 40A
without knowing how the 6mm cable was routed. I personally feel
uncomfortable using 6mm cable with anything more than a 32A breaker in
any case, because you just don't know what might happen in the future
(e.g. someone slaps a load on insulation on top).

--
Grunff

Andy Wade wrote:
Grunff wrote:

Ok, good point. Still finding my way around the on-site guide.
Whoever put that document together could really do with some
instruction on data structuring.

Yes it's abysmal, and poorly indexed too. And _still_ contains a
reference to a shower cubical (/sic./) [3.6.1(v)] which appeared in the
previous (blue) edition. It follows the formal structure of BS 7671
much too closely and hasn't been structured with the users' needs in
mind at all. And the division of material between the body and the
appendices is bizarre in the extreme.


An On-Site Guide is definitely needed, but the UK would probably be a
safer place without that particular document. I'm a technical writer, so
don't get me started...

Oh dear. Too late. You'd all better stay off the M4 for the next few
hours.


--
Ian White
Abingdon, England

In message ,
gna03633 wrote:

Firstly thanks for the advice/comments

How sure are you that the "identical" replacement had the same power
rating as the previous one? I strongly suspect your replacement is
more powerful, as a fused one is likely to be at 30A, giving a maximum
of 7.2kW. Of course, it is possible to get bigger fuses, but they
aren't that common. Don't go fitting a bigger fuse unless the cable is
found to be OK for the larger current.

I had the Aquatronic 8500 for a couple of years, it had blown the fuse
once before and was cutting out all the time. I had the manual left from
the previous owner and checked inside the housing for the rating. I
wanted somthing I could replace myself, so bought the Aquatronic 2 from
Wickes - for £60 or so with 2 years warranty. Everything was in the same
place i.e water/cables etc so was a easy swap.


It may not have been ideal (see other replies) but I have to say that
putting 8.5kW on a 30A fuse is very common in my experience. The fact is
that 8.5kW (approx 35.5A @ 240V) isn't going to blow a 30A rewireable
fuse anytime this century - check out fig 3.2A in appendix 3 of BS7671
if you don't believe me. A 30A rewireable will only begin to think about
blowing for a steady-state current of 55A or more (5 second
disconnection is 87A).

Bear in mind too, all those who have been giving cable ratings for 6mm2
cable, that with rewireable fuses you have to derate everything to 0.725
times nominal (433-02-03) thus 6mm2 cable at method 1 which would
normally give 47A is only rated to 34A.


I was concious of the cable and looked along it length I can see (in the
loft) any size mention, nothing written so measuring the size looked OK.


Should I be concerned about this?. I can easily replace the fuse and
hopefully catridge, but it has scortched some of the bit the fuse
plugs into - can this be replaced.

I went to my local electrical place an bought a 40amp Wylex Catridge (in
the manual for the shower it says use a 45A an 6mm. I also got a
'cutting' of 6mm cable so I could compare. Ive taken off the shower cover
and unscrewed the cable and measured it against, what I got given. They
both look the same size, with multiple strands for the core and measure
around 5mm with my ruler.

So I pretty confident the cable is good and the correct size.


I'd be doubtful that a wrong rating of fuse caused the burning
originally described. As others have pointed out, this is far more
likely to be down to a loose connection somewhere. Putting a 40A
rewireable fuse in won't cure this and is likely to be (as I pointed out
above) well outside the rating of the cable, even at the most lenient
installation method. Ignore what I've said if you've actually installed
a cartridge fuse (looks like a big plug fuse and says BS1361 on it)
rather than a rewireable.


Yes, but fuse boxes always seem to do this. After a few blows, they
look like melted Mozzarella on a pizza. Consider replacing your
consumer unit with a modern one, with RCD and MCB protection.

I seem to remember that the fuse did seem quite loose and maybe that
didn't help.


I would echo Christian's advice - at the least consider adding a
separate CU containing an MCB for the shower even if you don't go for a
full new CU. Screwfix do a couple of nice little units with RCD (may not
be strictly necessary, but good backup) and MCB already installed -
69659 for example, though as this comes with a 50A MCB you'll either
need to change to 10mm2 cable or swap the MCB for a lower-rated one.

Oh yes, and while you're at it, do make sure your supplementary bonding
is up to scratch.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Make offer or buy all for $100.00 Shipping or COD not included.

Andy Wade wrote in message ...
N. Thornton wrote:

I'm concerned by most of the advice in this thread - not just the
above. There coule be a live to earth fault and no bathroom bonding
for all we know.

How do you deduce that from the symptoms described?

what we can deduce is that that is one of the possible scenarios. We
can not deduce which scenario is definitely the one, the shower
install will need to be tested to determine and correct the fault.

If the above scenario is what the OP has, which is possible, fitting a
45A fuse might result in death.


Regards, NT

gna03633 wrote in message 40.11...

I seem to remember that the fuse did seem quite loose and maybe that
didn't help.

that might be all it is - and it might not. Personally I'd want to
know before reapplying power and getting in the shower

Grunff wrote:

I was asking whether it would really be ok to fuse the circuit at 40A
without knowing how the 6mm cable was routed. I personally feel
uncomfortable using 6mm cable with anything more than a 32A breaker in
any case, because you just don't know what might happen in the future
(e.g. someone slaps a load on insulation on top).

The fuse rating's a bit irrelevant here, provided that it still provides
short circuit fault and earth fault protection. It's the _load_ current
in relation to the cable rating that matters. Increasing the fuse
rating from 30 A to 40 A won't increase the current flowing through the
cable and therefore won't increase the conductor temperature.

The higher rated fuse will result in a higher final conductor
temperature after fault clearance, but such heating is essentially
adiabatic and isn't significantly affected by thermal insulation round
the cable.

So it's the earth fault loop impedance you need to check, not what's
happening in the loft.

--
Andy

Martin Angove wrote:

Bear in mind too, all those who have been giving cable ratings for 6mm2
cable, that with rewireable fuses you have to derate everything to 0.725
times nominal (433-02-03) thus 6mm2 cable at method 1 which would
normally give 47A is only rated to 34A.

Not in this case: the 0.725 cable derating factor for circuits with
rewireable fuses only applies to circuits liable to be overloaded. A
shower circuit isn't liable to overload because you have a fixed load
resistance. Overload protection can be omitted [473-01-04(ii)] and the
433 group of regulations then doesn't apply. See also appendix 4,
section 6.3.

I'd be doubtful that a wrong rating of fuse caused the burning
originally described.

I'd say it can certainly contribute though. 35 / 30 is 17% over the In
of the fuse, leading to 36% excess power dissipation in the fuse
assembly (more in fact because the resistance of the fuse wire will
increase when it's hot). The temperature rise of the fuseholder parts
could then be, say, 40% more than was intended - probably enough to
contribute to the slow cooking.

As others have pointed out, this is far more
likely to be down to a loose connection somewhere. Putting a 40A
rewireable fuse in won't cure this and is likely to be (as I pointed out
above) well outside the rating of the cable, even at the most lenient
installation method. Ignore what I've said if you've actually installed
a cartridge fuse (looks like a big plug fuse and says BS1361 on it)
rather than a rewireable.

I beg to disagree. As I said in reply to Grunff, increasing the fuse
rating won't affect how hot the cable gets. Yes, the cable installation
conditions should be checked as part of the process of putting this
right - but in relation to the actual load current (the design current
of the circuit Ib, modified by any grouping and/or ambient temperature
correction factors that apply) not the fuse rating. The important thing
is to check the earth fault loop impedance to ensure that, with the
larger fuse, 5 s disconnection time is still met, and for adiabatic
compliance of the CPC.

[Screwfix CU] though as this comes with a 50A MCB you'll either
need to change to 10mm2 cable or swap the MCB for a lower-rated one.

I'd swap the MCB anyway, but again, in a circuit not liable to overload,
the cable rating required can be determined by the load current, not the
protective device rating. There may be no need to replace the 6mm^2,
provided you're prepared to do some sums.

--
Andy

N. Thornton wrote:

Andy Wade wrote...

N. Thornton wrote:

I'm concerned by most of the advice in this thread - not just the
above. There coule be a live to earth fault and no bathroom bonding
for all we know.

How do you deduce that from the symptoms described?

what we can deduce is that that is one of the possible scenarios.

Sorry but it isn't. A fault (whether phase-earth or phase-neutral)
would blow the fuse quickly -- or if it didn't, due to gross
miscalculation of the circuit design, it would set fire to the cable
instead. Either way, the shower would be rendered inoperative.

OTOH the symptoms described here by the OP seem to me to be those of a
'slow cooking' of the fuse, probably over a lengthy period. This is
entirely consistent with a diagnosis of some combination of an
under-rated fuse and loose connection(s).

--
Andy

Andy Wade wrote:


I'd swap the MCB anyway, but again, in a circuit not liable to overload,
the cable rating required can be determined by the load current, not the
protective device rating.

Amen to that! I think we need to get some T-shirts printed with that
on the front, and the full wording of 473-01-04(ii) which you mention,
and ship them out to the "uprate everything"
I-can't-believe-I'm-NICEIC-registered gold-braid brigade ;-)

Stefek

Stefek Zaba wrote:

Amen to that! I think we need to get some T-shirts printed with that
on the front, and the full wording of 473-01-04(ii) which you mention,
and ship them out to the "uprate everything"
I-can't-believe-I'm-NICEIC-registered gold-braid brigade ;-)

:~)

--
Andy

Andy Wade wrote in message ...
N. Thornton wrote:

I'm concerned by most of the advice in this thread - not just the
above. There coule be a live to earth fault and no bathroom bonding
for all we know.

How do you deduce that from the symptoms described?

what we can deduce is that that is one of the possible scenarios.

Sorry but it isn't. A fault (whether phase-earth or phase-neutral)
would blow the fuse quickly -- or if it didn't, due to gross
miscalculation of the circuit design, it would set fire to the cable
instead. Either way, the shower would be rendered inoperative.

I can be quite clear that that is not correct. Fuses require large
overloads for long times to blow. An L to E leak that took a
significant time to blow might be present. Fitting a new fuse and
getting under such a shower would likely kill.


OTOH the symptoms described here by the OP seem to me to be those of a
'slow cooking' of the fuse, probably over a lengthy period. This is
entirely consistent with a diagnosis of some combination of an
under-rated fuse and loose connection(s).

They are, but that is not the only scenario with which the description
is consistent. It would be different with a portable appliance, but
we're talking an electric shower here. If it were mine I would
insulation test it first.


Regards, NT

N. Thornton wrote:
Andy Wade wrote in message ...

Sorry but it isn't. A fault (whether phase-earth or phase-neutral)
would blow the fuse quickly -- or if it didn't, due to gross
miscalculation of the circuit design, it would set fire to the cable
instead. Either way, the shower would be rendered inoperative.

I can be quite clear that that is not correct. Fuses require large
overloads for long times to blow. An L to E leak that took a
significant time to blow might be present. Fitting a new fuse and
getting under such a shower would likely kill.

You've moved your own goalposts. Now you're postulating a 'leak' that
would overload the circuit enough to cook the fuseholder, but not blow
the fuse quickly. Such a leak would be passing several amps and would
dissipate lots of heat, thus drawing attention to itself - unless the
leakage was purely reactive, but what could cause that? I suppose that
what could happen is an earth fault part-way along the shower's heating
element, turning the 8.5 kW shower into a 10 kW one - pretty unlikely
though.

They are, but that is not the only scenario with which the description
is consistent. It would be different with a portable appliance, but
we're talking an electric shower here. If it were mine I would
insulation test it first.

I would too, as a precaution, and also check the load current (easy if
you've got a clamp-meter). But, IMHO, checking the tightness of
connections, the earth fault loop impedance and the supplementary
bonding are all more important if you are worried about being
electrocuted under the shower.

--
Andy

Andy Wade wrote in message ...
N. Thornton wrote:

Sorry but it isn't. A fault (whether phase-earth or phase-neutral)
would blow the fuse quickly -- or if it didn't, due to gross
miscalculation of the circuit design, it would set fire to the cable
instead. Either way, the shower would be rendered inoperative.

I can be quite clear that that is not correct. Fuses require large
overloads for long times to blow. An L to E leak that took a
significant time to blow might be present. Fitting a new fuse and
getting under such a shower would likely kill.

You've moved your own goalposts. Now you're postulating a 'leak' that
would overload the circuit enough to cook the fuseholder, but not blow
the fuse quickly. Such a leak would be passing several amps and would
dissipate lots of heat, thus drawing attention to itself - unless the
leakage was purely reactive, but what could cause that? I suppose that
what could happen is an earth fault part-way along the shower's heating
element, turning the 8.5 kW shower into a 10 kW one - pretty unlikely
though.

I think youll find such a fault is normal and common. Its how millions
of immersion heaters fail, they split open but keep going for a while,
with current flowing through the water, and thus total load higher
than it should be. The supply wiring gets hot but keeps going. It can
take a very long time for a wire fuse to blow - in some cases like a
whole year.

Nothing is obviously wrong, it continues to function, and if earth
impedance is low, as is usually the case, no-one is any the wiser.

If the install is modern with rcd and bonding, this will not be a real
safety problem, but if it has neither of those, the OP might possibly
be in for a very nasty surprise.


They are, but that is not the only scenario with which the description
is consistent. It would be different with a portable appliance, but
we're talking an electric shower here. If it were mine I would
insulation test it first.

I would too, as a precaution, and also check the load current (easy if
you've got a clamp-meter). But, IMHO, checking the tightness of
connections, the earth fault loop impedance and the supplementary
bonding are all more important if you are worried about being
electrocuted under the shower.

I agree, either approach would ensure safety.


Regards, NT