HELP WITH "SLOW BLOW FUSE" AND CONFUSING SITUATION

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Hi

Yesterday I had to replace a Grundfos circulating pump on a Combi Boiler.

The new pump was a direct replacement unit.

I switched off the 3 amp fused spur.

The cable/current/load for the 240v pump as far as I can see does NOT go through the boiler PCB (the computer board).

To connect the 3 wires to the pump I pushed the three soldered ends of the cable through a small hole (protective rubber piece),
as soon as all the wires touched the consumer unit TRIPPED.

I was always under the impression that that a fused spur has "a gap" when the switched is turned off.

But I checked the 3 amp fuse anyway, and this fuse HAD NOT blown.

So I pulled out the Slow Blow fuse that is positioned on the incoming side of the mains power. This slow blow fuse is positioned right
next to the PCB and I'm just guessing, but I would think this is to protect the PCB.

Holding the slow blow fuse up to the light that has a "clear glass" so you can see the fuse wire, the wire looks intact.

The slow blow fuse has "red stripes" going around the glass but one of the stripes is GRAY/BLACK - now I don't know if
the gray/black stripe has always been there or it is an indication that the fuse has blown.

Due to a serious problem with the connections block on the pump, the pump has got to be returned, so I wasn't able to
test the boiler to see if it was going to work.

The problem on the pump was down to the quick fit (no screws to connect wires), just push small orange plastic bit
with screwdriver to open each connector, to push soldered end of wire in, take pressure of screwdriver to close
the connector.

Unfortunately the middle block for 'live' was broken and I could not open the connector. I tried for hours to connect but had
to give up in the end, as the plastic bit was just floppy and there was no way to get at it to try and fix. The connector block
was fixed tight to the printed circuit board, that made it impossible.

So two questions;

If the spur fuse did not blow how/why did the consumer unit trip?

If the Slow Blow fuse has blown, would a slow blow fuse actually trip the consumer unit?

I will buy another slow blow fuse to be on the safe side, but do you know where I can get these type of fuses.

I haven't got the fuse with me but from memory I think the manual says it's 2 amp.

It is half the size of a normal 13 amp plug.

Any help or advise would be appreciated, as I really hope I haven't damaged the boiler.

Confused and now worried.

Many thanks.

Stephen
 
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Shorting the neutral to the earth will cause the RCD to trip if power is being used elsewhere on a different circuit protected by the same RCD even if the MCB ( or the FCU switched spur ) is turned OFF.

This is normal

There is a gap when OFF but only in the Live, the Neutral remains connected.
 
The slow blow fuse has "red stripes" going around the glass but one of the stripes is GRAY/BLACK - now I don't know if
the gray/black stripe has always been there or it is an indication that the fuse has blown.
The coloured bands could be a code to show the type of fuse.

http://highfields-arc.co.uk/constructors/info/fusecolours.htm


If the Slow Blow fuse has blown, would a slow blow fuse actually trip the consumer unit?
No. But you can check if the fuse has blown with your multimeter.


I will buy another slow blow fuse to be on the safe side, but do you know where I can get these type of fuses.
Farnell/Rapid Electronics/Maplins/eBay/Amazon/ http://www.google.co.uk/search?q=slow+blow+fuses&tbm=shop ....


Any help or advise would be appreciated, as I really hope I haven't damaged the boiler.
You are sure it's a slow-blow fuse, if you do replace it?
 
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Clearly a single-pole FCU.
It certainly sounds as if it is, but I have to say that I don't recall having come across such an animal. IME, even the cheapest ones these days seem to be DP. Was there perhaps a day when single-pole ones were around (maybe even common)?

Kind Regards, John.
 
Although some glass fuses are 250 volt rated, i cant recall the last time i see one used as such, are you sure it wernt load side of a transformer or similar
 
So I pulled out the Slow Blow fuse that is positioned on the incoming side of the mains power. This slow blow fuse is positioned right next to the PCB and I'm just guessing, but I would think this is to protect the PCB.
PCBs can die extremely rapidly - so I'd be pretty surprised if anyone would use a slow-blow fuse in an attempt to protect one! What makes you think/know that it is a slow-blow fuse?

Kind Regards, John
 
PCBs can die extremely rapidly - so I'd be pretty surprised if anyone would use a slow-blow fuse in an attempt to protect one!
I wondered that. My initial thought was "surely you'd want a fast acting fuse to protect a PCB?".
But... http://www.google.co.uk/searchq=boiler+pcb+fuse+slow+blow[/QUOTE]
I don't think that's really a 'but'. It sounds as if you and I are agreed that it would make no sense to have a slow-blow fuse "to protect a PCB" - so that's probably not what it's there for (I frankly doubt that even an ultra-fast fuse would protect a PCB!). If the fuse is is the supply input to a boiler (particularly if a combi), (to protect cabling within the boiler or whatever) then the inrush current of pumps, solenoids, motorised valves etc. might well require it to be slow-blow fuse in order to avoid it having to have an In which was so high as to be counter-productive.

Kind Regards, John
 
:confused:

I FORGOT TO ADD ON LAST POST SOMETHING THAT IS IMPORTANT

So I've added it to this original post, to save explaining it all again.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When I went to disconnect the OLD pump I saw that the LIVE and NEUTRAL wires were
connected the wrong way round (live in neutral connector).

I was going to connect as the original pump was wired up i.e. the wrong way round but
then I thought if I do this and it blows the new pump I wouldn't be able to send the pump
back as faulty.

I'm sure somewhere long ago that I read that swapping the 'neutral and live' around in certain
situations this is possible - but I didn't want to take the chance.

So as it stands the pump company were amazing after I emailed them the problem about the
pump - that is the orange faulty connectors.

The new pump arrived this morning (they collected the faulty pump) BUT it has come supplied
with a long "suppression lead".

This long lead will make it easier to wire up the pump away from boiler but this throws up another
question.

So I'd appreciate just a few more answers;

1) Should I follow the WRONG way round wiring i.e. live to neutral connection and neutral to live
connection OR WIRE UP as it should be i.e. neutral to neutral etc?

2) When I connect the long suppression lead to the main cable coming from the boiler do I just
use a plastic connector block and wrap some electrical tape around it?

3) I have got some CHOC BLOCKS for Halogen lighting would this be ok to use?

4) Does anyone know what this suppression lead does?


Many thanks for everyone that has helped.

Regards

Stephen
 
Just to say thanks to all.

The onlinepump site were so helpful and delivered a replacement pump the next morning.

Unfortunately, this pump came supplied with a suppression lead and no help with what to do with regards to boiler end cable.

I phoned Grundfos today to double check as I'd already connected to a CHOC Box will block inside, just to see if that was okay - they said yes.

Technical support said although this was a new pump - the suppression lead is old technology (it controls voltage to pump).

Anyway I fitted the pump head, connected the cable the right way round (live to live etc - as the old pump had been wired up back to front).

Although the Alpha slow blow fuse (FA2) was okay I'd brought two more at £1.40 each but next day delivery was £25 - yes £25 - I paid £10 and still got next day.

Put in new fuses, filled up the system and the boiler still didn't work. I checked all the connections best I could (no meter), but everything points to PCB - it is not worth paying out for a new PCB (boiler is to old) so I'll try and sell pump on eBay and now we'll have to buy a new boiler.

It was the worth the gamble but I lost on that one.

Thanks again.
 
AC is alternating - it doesn't have a wrong way round.

For a simple device with two connections (like this pump), it'll work just the same whichever way round it is connected. It'll need to have insulation for the required voltage so even the "neutral is closer to earth" argument is moot.

However, it's bad practice not to do it right.
 
:)


Hi

First chance to get back and wrap this post up.

Just thought I'd add what the final outcome is going to be for those
that have followed other posts connected to this one - purely to help
anyone else who might find themselves in a similar situation.

I have included this post to the others posts connected.

I looked in to other boilers that others recommended.

I also checked other web sites with Boiler Installers recommendations
who on balance rated Worcester Bosch Greenstar **CDi Classic
as the boiler to go with.

Briefly a while back I did a flow rate test - 10 seconds gave me two litres.

So this equated to 12 litres per minute.

The boiler to be replaced was an Alpha 28CB.
The 12 litres per minute would have matched the Alpha.

I nearly went for Greenstar 29CDi Classic LPG - because that was the nearest
to the Alpha . . . . BUT I thought I best double check flow rate again.

This time I did a FULL 60 SECONDS and this resulted in exactly 14 litres per minute.

I really don't understand this discrepancy, (12 vs 14) but this is what it came out as.

So I phoned Worcester Bosch main help line - the woman didn't have a clue what I was asking
about and put me through to Technical dept.

The guy said the two boilers that I thought would be okay the
Greenstar 34CDi Classic OR Greenstar 38CDi Classic were both okay with 14/lpm

He said he'd go for the "38CDi" because it delivers 14 litres per minutes at 40 degrees.

And because it's not more than around £150 I'm going to go for the Greenstar 38CDi Classic LPG

I asked about an installer, he said if you use a local Gas Safe guy I wouldn't receive 7 years
(8 with fitted filter) parts & labour guarantee BUT I would get 5 years parts & labour guarantee.


For those interested Greenstar info;

Greenstar 34CDi Classic

Central heating output: 30kW
Domestic hot water flow rate: 14.3 litres per minute at 35 degree temperature rise
(provided adequate mains pressure is available)
Designed operational flow rate 12 litres per minutes at 40 degree temperature rise.


Greenstar 38CDi Classic

Central heating output: 30kW
Domestic hot water flow rate: 16.4 litres per minute at 35 degree temperature rise
(provided adequate mains pressure is available)
Designed operational flow rate 14 litres per minutes at 40 degree temperature rise.

Many thanks again - I couldn't have got here eventually without helpful snippets here and there.

Stephen
 

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