Electric shower cutting out after 4mins - cannot diagnose problem!!

Ell_ma

I have a Mira electric shower in my flat (9kw). I have had the same model of shower for six years. The first unit lasted for five years. The shower then conked out entirely and I was told that electric showers don't live that long so this wasn't unexpected and I replaced it with a new (like for like) Mira unit. This was fine for six months and but it then developed a fault so now when used on "High" or "High + Air Boost" (which is really the only setting that gives an decent power) it only lasts for 4mins then the power on the unit cuts out and the light on the pull cord also goes out. If you wait a few mins then pull the pull cord to "off" then pull again to turn on then the light comes back on on the pull cord - then the shower after a few secs - and you can run the shower again for another 4mins before the same thing happens again. I've had Mira out and they have replaced with a new unit (again like for like - 9kw) but this has not solved the problem. I have also had an electrician out who has tested the fuse box, run a "continuity test", "insulation resistance test", "voltage test" etc. (all their words) and found nothing wrong with either the fuse box or the circuit from fuse to pull cord, pull cord to shower. The switch on the fuse box itself doesn't trip when the shower goes off. Essentially noone can tell me what the problem is so i'm wondering if it could still be something to do with the pull cord/pattress box (I believe this is what it is called!). Should I try replacing that as a next step even though I understand that the pull cord doesn't have a fuse in it(?) - could there been an issue in the pattress box that only happens on full power and doesn't show on a test?!
I'm completely baffled - the only other option that has been suggested is to replace the entire fuse box @ around £800!!

Any thoughts?!

Le_Kirk

Given the fault finding that has already been conducted, my next port of call would be the pull switch - it is the only thing that hasn't been checked or changed.  Have to say I would be pretty teed off if I was Mira and I fitted a new shower due to a faulty switch.  Also what the electrician doing; not checking an obvious link in the chain?

Risteard

Sounds like a loose connection. This is a serious fire risk.

Apodemus

Have you looked at the water pressure?  Sounds like it copes with a certain amount of heating then goes to an overheat lock-out, which re-sets once it cools down.  If all the electrics are OK and it is occurring on multiple identical machines, then it may be that the water pressure is marginally too low?  I presume it is fed directly from the mains, rather than from a header tank?

EDIT:  I see that it only happens on High, which is what you would expect if it was an overheat cut-out issue.

FreeBear

It could also be a partial blockage in the shower head - Give the head a good soak in a descaling solution and also clean any filters in the hoses.

Apodemus

Actually, thinking further about this, the pull switch is surely wired up wrongly, if the indicator light is able to go out, when the shower cuts out?  This may be the primary problem or simply a secondary issue, but, either way, it needs fixing. 

Risteard

Apodemus said:

Actually, thinking further about this, the pull switch is surely wired up wrongly, if the indicator light is able to go out, when the shower cuts out?  This may be the primary problem or simply a secondary issue, but, either way, it needs fixing. 

This is indicative of a loose connection where the load is causing the light to go out.

Ell_ma

Apodemus said:

Have you looked at the water pressure?  Sounds like it copes with a certain amount of heating then goes to an overheat lock-out, which re-sets once it cools down.  If all the electrics are OK and it is occurring on multiple identical machines, then it may be that the water pressure is marginally too low?  I presume it is fed directly from the mains, rather than from a header tank?

EDIT:  I see that it only happens on High, which is what you would expect if it was an overheat cut-out issue.

Thank you for your responses! - i'm just baffled why water pressure would suddenly become an issue after all these years? Mira didn't seem to think it was a water pressure issue - the only person that's mentioned that was the electrician...... If it is a water pressure issue then I could try a shower with integrated pump but again that's a big cost without knowing whether that's the problem.

FreeBear

Ell_ma said:  If it is a water pressure issue then I could try a shower with integrated pump but again that's a big cost without knowing whether that's the problem.

Generally, pumped electric showers are run from a header tank (usually in the loft), and most are not recommended for connection directly to mains water.

If the Mira is anything like my shower (a Triton), there is a warning light that comes on when water pressure is low.

Jeepers_Creepers

The evidence here points directly at an electrical supply fault as Risteard says.

The fact that the indicator light in the pull switch goes off shows that the power has stopped before it reaches that light. Almost certainly your actual shower is fine - it does, after all, work perfectly fine for these 4 minutes.

The most common cause is the pull switch itself - the screw terminals for the wires are not tight enough so the connection with the wire starts to arc and go high resistance. Usually it becomes hot too, and it's very common to find the plastic insulation meted around the miscreant wire.

Since the light also goes out, this points to the 'supply' side of the switch - the cable coming from the CU. The same issue - a loose terminal/high resistance connection - could also have happened at the CU end, and potentially even inside the MCB (assuming that's what you have). 

So, first check (with POWER OFF) is the pull switch - unscrew and have a gander inside. Look first for signs of overheated wires, and then check the terminal tightness, whilst also making sure the wire is firmly inserted right in, has been 'bared' and has not been tightened down on the insulation.

If that is clearly ok, then check the CU and look at the terminals on that MCB and also on the Neutral wire for the shower.

If both of these draw a blank, then are you 100% certain there isn't a JB along the cable somewhere that could be ditto?

Finally, if none of that has comes up as the cause, it's time to replace the pull cord switch first, and the MCB afterwards.

There are other checks you can do, but it involves testing for voltages with the power on - eg running the shower with the pull switch loosened, and when the shower and neon goes off, checking for a mains voltage at the supply end of the switch. If the voltage has disappeared here, then repeat at the CU - is there a voltage on the output of the MCB?

oh_really

Thermal damage at the pull cord switch.