wrote:
What length is the cable run?
up to 15 metres (10mm cable)
OK, we probably have enough info to try a design exercise...
(see end)
Will it be fed from a RCD protected supply?
An RCD is a device that greatly reduces the danger posed by an electric
shock. See http://wiki.diyfaq.org.uk/index.php?title=RCD
What protective device do you plan on using (fuse / MCB etc)?
Not well up on this - was going to get a sparky to plumb it into my
fuse box - the highest rated breaker in the box is 45A so I'm guessing
he will fit a separate 60A breaker/isolation switch or whatever is
necessary
A 45A type B MCB will be OK. Depending on circumstances, you may want to
use a separate enclosure with a RCD, or you could add the MCB to the RCD
protected side of your CU is it is a split load type, or you could add a
RCBO (combined MCB and RCD) to the CU.
Types of CU explained he
http://wiki.diyfaq.org.uk/index.php?...r_Beginners#CU
What earthing system does your house have?
I have no idea. I thought all houses were earthed the same
No, there are two common ways for properties in towns, and a third
common way for properties out in the sticks. See:
http://www.diyfaq.org.uk/electrical/....html#earthing
Given the above you can work out the prospective fault current at the
shower and hence the disconnection times, and you can also check the
earth fault loop impedance. You will also need to check the maximum
voltage drop is not exceeded.
Okay, now I'm lost.
"Prospective fault current" is a figure that tells you how much current
can flow in a circuit when things go badly wrong and you end up with a
short circuit at the end of it. It will be limited only by the
resistance of the cables in the circuit and the resistance the supply
itself and also of the path to earth (in the case of a fault to earth).
It needs to be large enough to cause the protective device (i.e. fuse /
MCB) to open quickly to minimise the risks of shocks to you, and also to
prevent damage to cables from overheating.
You need an isolation switch - preferably in the same room but not
somewhere that can be reached from in the shower.
I was going to fit a pull switch isolation switch in the bathroom
which cannot be reached from the shower but through which it's mains
supply will obviously pass.
Yup, that's fine. Get a decent quality one, because 10mm^2 cable is a
pig to work with and can put lots of strain on plastic accessories - so
a strong one with good quality terminals is worth buying.
You suplimentary equipotential bonding will need to be up to scratch
unless you plan to implement 17th edition style 30mA RCD protection for
all circuits in the shower room.
Meaning?
The traditional way of mitigating the danger of electricity in bathrooms
etc is via equipotential bonding. This is done by joining together all
the bits of metal that have the potential to introduce a voltage into
the room. So pipes, the earth wires on all circuits, and a few other
things are all wired together with heavy gauge earth wires. It means
that if during a fault end up with a dangerous voltage on something, it
is duplicated on *everything* in the room that conducts. This means that
you can't simultaneously touch two things at a markedly different
potential, and hence get a shock.
The new edition of the wiring regs (the 17th edition) does away with the
need for bonding, and instead insists that all circuits in a bathroom
are protected by RCDs with trip currents not exceeding 30mA. The RCD(s)
will instead limit any potential shock duration to a safe limit. Note
following the 17th edition is not mandatory until the summer this year.
Thankyou for the advice - although you've lost me a little on some of
this
Sorry about that - there is a fair bit of complexity when designing and
installing circuits like this. In many cases even if you don't know all
the detail there is a fair chance you will get away with it and end up
with something that is safe, but, the proper way to do it is to do a
design exercise first and prove this is the case. (Note you also ideally
need copies of BS7671 and the On Site Guide to be able to look up all
the cable and protective device performance data (a fair bit of what you
need is reproduced in our wiki))
For example, using what you have told us so far we can do the following:
(design calcs)
Your shower is a 10.8kW one. That is probably the spec at 240V, so that
suggests an actual current of 10800 / 240 = 45A. We can compute the
resistance of its heating element as 10800 = 240^2 / R, so R = 5.3 ohms.
For design purposes we work at 230V. So the likely power output at that
voltage will be 230^2 / 5.3 = 9981W. The design current is hence 9981 /
230 = about 43A.
So a 45A breaker would be OK.
The cable (10mm^2 cable clipped direct), is good for 64A, so we can
safely say that is adequate.
Voltage drop
see table :
http://wiki.diyfaq.org.uk/index.php?...s#Voltage_Drop
is 4.4mV / A / m, and so that gives us 0.0044 * 43 * 15 = 2.8V which is
less than the 4% allowed, so that is ok.
Lets look at disconnect time for a phase to earth fault at the shower.
We need to open the protective device within 5 seconds (radial circuit
supplying fixed equipment inside an equipotential zone)...
First thing we need to know is the maximum current that might flow under
fault conditions. For this we need the total resistance of the circuit
and its supply and earth connections. Some bits we can calculate:
http://wiki.diyfaq.org.uk/index.php?... istance_table
So the round trip (phase + earth) resistance of the cable is 6.44 mOhms
/ meter or 0.00644 * 15 = 0.0966 ohms. If we assume for a moment that
your house has a TN-S[1] earthing system, we can use a worst case earth
fault loop impedance at your consumer unit of 0.8 ohms, so we have a
total loop impedance of 0.8996 ohms. Now that assumes normal operating
temperature; to allow for running at close to cable maximum capacity
when the fault occurs we introduce a correction factor of 1.2, giving a
resistance of 1.07952 ohms.
The fault current would therefore be 230 / 1.07952 = 213A. A look at
figure 3.4 in the wiring regs shows that a 50A[2] type B MCB will open
"instantly" (i.e. in 0.1 secs) for fault currents of 250A or more, and a
40A device at 200A or more. That suggests your 45A breaker is going to
be borderline. So you may need a RCD in there[3] to ensure disconnection
on an earth fault. In reality there is a fair chance that your actual
earth fault loop impedance will be less than the 0.8 ohm figure we
assumed (you would need test equipment to measure this), and the voltage
and therefore fault current will be higher than that calculated - so you
would probably get away without without a RCD. With a TN-C-S supply you
would be fine also since the worst case earth loop impedance is 0.35 ohms.
[1] See
http://www.diyfaq.org.uk/electrical/....html#earthing
Assuming TN-S is more pessimistic than assuming TN-C-S - if you actually
have TT then you will have to use a RCD regardless.
[2] No curve shown for a 45A device - but we can safely assume that will
trip instantly at a lower current than a 50A device. We can also
interpolate between the figures given for the 40 and 50A devices.
[3] Many electricians will fit a RCD on a shower feed as a mater of
course these days, and also the coming 17th edition will mandate it.
Lets assume (in the absence of real measurements), that we are ok on
disconnection time. We need one final check that the earth wire in the
cable will withstand that magnitude of fault current for long enough to
allow the circuit breaker to operate. So we compute the minimal cross
sectional area "s" of the earth wire using the adiabatic equation s =
sqrt( I^2 x t ) / k. For PVC clad cable, k = 115, so we get sqrt( 213^2
x 0.1 ) / 115 = 0.59mm^2 which is comfortably less than the 4mm^2 earth
wire in the 10mm^2 cable. So that is ok.
(note its late, and I wrote that quickly - so don't rely on it as gospel
without checking!)
--
Cheers,
John.
/================================================== ===============\
| Internode Ltd -
http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/