Isolator Query

[wreck-it-steve]

I currently have a SWA cable running outside powering two pumps and if there is ever a fault it blowes a 13A fuse in a fused spur. I want to get rid of this and wire it directly to its own MCB. My question is: there is already an isolator on the main control panel for the pumps outside but would it be wise to put an isolator indoors after the MCB?

Thanks

 

[PrenticeBoyofDerry]

It would be wise to have an isolator as close to the equipment as practical, what is the set up, do you have selv trany and RCD protection?
If the fuse is blowing you have a fault, that should be the first thing you should be investigating.
Is the the isolator correctly IP rated for the environment it is in?
You say SWA cable is routed outside, where is the power supply coming from, directly from the Consumer unit or is it a spur of an existing circuit within the property?

 

[wreck-it-steve]

It would be wise to have an isolator as close to the equipment as practical.
If the fuse is blowing you have a fault, that should be the first thing you should be investigating.
Is the the isolator correctly IP rated for the environment it is in?
You say SWA cable is routed outside, where is the power supply coming from, directly from the Consumer unit or is it a spur of an existing circuit within the property?

Their is an isolator next to the equiptment outside that is rated at IP66 but i just wondered if it would require on indoors also next to the CU as I have been told in the past never to use the MCB as an isolation device. When I say the fuse blows in the FCU i mean every now and then due to load. Thats why everything has been altered now to accomodate for this. It is all fed from the CU on its own MCB.

Thanks

 

[ericmark]

There are special MCB's designed for motors. These are multi-pole and have a range of overload values so it can be matched to the motor. They also have holes in the knob or push button so they can be locked off. See here for examples

However before moving from fuse to MCB of any type you will need to measure the loop impedance. If will look at the standard MCB it is made of two bits working together. One is thermal the other is magnetic. The magnetic bit of the MCB has a fixed current required to work it so unlike the fuse which if the loop impedance is a little too high just takes a little longer to trip. With the MCB if the loop impedance is too high the trip time can jump from milliseconds to minutes.

So again with the standard MCB we have three letters. B = 5 times, C = 10 times and D = 20 times the rated current for the magnetic part to work.

So for a D16 MCB you need 320 amp to flow before it will trip the magnetic part. So the loop impedance has to be less than 0.72 ohms. So step one is measure the loop impedance.

There are two loop impedances one line to neutral and one line to earth. The latter's requirement changes when a RCD is used but not the line to neutral impedance.

I linked to the RS web site as they have all the data sheets and you will need to read them to work out what loop impedance will be required when using special MCB's.

If you don't have a loop impedance meter then at £75 to even hire I would not attempt to DIY the job.

 

[PrenticeBoyofDerry]

I have been told in the past never to use the MCB as an isolation device.

By who?

When I say the fuse blows in the FCU i mean every now and then due to load. Thats why everything has been altered now to accomodate for this. It is all fed from the CU on its own MCB.

in what way?
Have you changed the fuse rating, is the cable still suitable for this?

 

[bhm1712]

if a fuse is blowing due to load, it means the circuit and protection isnt suitable for the load's connected to it!!

The isolator should be next to the equipment as it already is, and it is perfectly ok,as stated before, to use an MCB as a method of isolation provided it can be locked off in some way, in your case you have 3 forms of isolation already....the isolator, the fuse in the FCU and the MCB supplying the circuit.... if i was going to be working on the pumps i would lock the isolator off and pull the fuse in the FCU, and remove the tails from the FCU at the same time.

 

[OwainDIYer]

I have been told in the past never to use the MCB as an isolation device.

By who?

Most MCBs in UK consumer units are single pole, therefore do not disconnect the neutral, and are therefore not suitable as a means of isolation.

 

[bhm1712]

ooopps! yes totally agree, im used to double pole or 4 pole mcb's at work (industrial!)

However, in this situation, he has a local isolator which coupled together with tails out at the FCU and proven dead, is acceptable

 

[wreck-it-steve]

So cable direct in to MCB (1.5mm on 16A MCB) and local isolator at panel is OK?

 

[bhm1712]

in terms of safe isolation , then yes the local isolator is acceptable. However you say there is a fuse in a FCU which blows. Is this supplying the pumps?

1.5mm is rated upto 15 amp if i remember correctly, but thats if its clipped direct. Does your cable travel through walls, under concrete etc etc??

what are the pumps doing? do they run all day long? just for 20 mins a day....?

if you tell us the exact layout of the circuit we can help more.

As far as i understand from what you say its as follows:

16A MCB in CU--------1.5mm-------------FCU with 13amp fuse --------1.5mm SWA-------------Local Isolator------Pumps.

Is that right? what is the 16A mcb also feeding?

 

[wreck-it-steve]

in terms of safe isolation , then yes the local isolator is acceptable. However you say there is a fuse in a FCU which blows. Is this supplying the pumps?

1.5mm is rated upto 15 amp if i remember correctly, but thats if its clipped direct. Does your cable travel through walls, under concrete etc etc??

what are the pumps doing? do they run all day long? just for 20 mins a day....?

if you tell us the exact layout of the circuit we can help more.

As far as i understand from what you say its as follows:

16A MCB in CU--------1.5mm-------------FCU with 13amp fuse --------1.5mm SWA-------------Local Isolator------Pumps.

Is that right? what is the 16A mcb also feeding?

It currently is:

16A MCB in CU--------1.5mm-------------FCU with 13amp fuse --------1.5mm SWA-------------Local Isolator------Pumps.

But I want it to go:

16A MCB in CU--------1.5mm-------------1.5mm SWA-------------Local Isolator------Pumps.

as the new pump control panel has MCB's for each pump. The SWA runs clipped to the wall and then under ground in ducting. The pumps only come on when their float switch goes above the set level.

 

[bhm1712]

is this a Grundfos Multi-E booster set by any chance?

What is the full load current of the pumps?

what your proposing appears ok depending on the above.