How can i trip an RCBO

Postby collectors » Thu May 19, 2011 5:56 am

Got a rain water system that has a pump on it. It all works great with float switches & 12 volt relays. What i would like is a secondary fail safe way of tripping the RCBO in case the water level gets to high in a storage tank. Is it ok to have a 12 volt float switch controlling a relay that shorts out the earth & neutral via volt free contacts on the RCBO.
Or do you have another suggestion???.


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Postby ericmark » Thu May 19, 2011 3:30 pm

Larger RCD units do have a remote option but in the main these are for where more than 100A is being controlled by the RCD.

Some RCBO and MCB devices are designed to link with other units. In which case the second unit can trip them. These vary from make to make. Although used in distribution units never seen one in a consumer unit and they may not comply with the type testing required.

Either way you are looking at some specialist applications. To try and trip an RCD with a earth / neutral link would be rather hit and miss. If the earth and neutral are at the same polarity then linking them will cause no current to flow so it would not trip. For the polarity to be different with a TN-C-S system there must be some current draw. So the operation would depend on the current being used at the time. Clearly not fail safe.

The normal way to control filling and empting of tanks where some thing more than simple floats are required is to use a PLC. Very clever people may use a PIC which requires more skill to program but cost less to buy. Your washing machine is likely controlled by a PIC as is the engine management in your car.

The problem with any secondary system is testing it works. There have been some really bad accidents caused testing that secondary systems do work. It caused one of the biggest nuclear accidents. OK your not going to get to that stage but I am sure you can see the problem.

Using a second float would be common but at some time the first float has to be disconnected to test second one.

Using a PLC the common method is time. So for example when bottom tank fills pump is run to transfer into top tank which should take less than 1 hour so if runs for more than 1 hour system closes down.

Circuit diagrams can't be posted in this forum. But you can on others. You need to explain what the problem is and maybe there is a method to get around it. But shorting earth and neutral will not be a valid method.
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Postby collectors » Thu May 19, 2011 6:26 pm

Many thanks ericmark & points taken. I might have a look down the motor over load relay/starter switch route with a suitable amperage for the pump to run & possibly using the same method as above introduce an extra load that will trip the overload relay/starter switch.
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Postby ericmark » Fri May 20, 2011 9:58 pm

There are basic two types of motor overload.

The type similar to a MCB are designed to remove the need for a MCB with the overload doing both jobs. They are not cheap but can be locked off so I used them a lot in boards I built.

The other type is a sensor with contacts and these in turn work the contactor. With this type all sorts of extras can be added. Float switches being quite common.

Before I got into programming PLC's I did a board to run two pumps with a series of floats so as water level started to rise first small pump ran, then large pump ran instead and finally both ran.

The whole idea of tripping an overload by giving it an overload seems rather pointless.

I have used current sensing relays this was to stop a plant running unless the extractor was on. It also auto rotated between the two extractors and auto swapped if one failed. Again all linked into PLC.

But using an overload as a current sensing relay does not seem a good idea as the device is thermal and could over heat.

Maybe if you explain what you want better some one on here will have a easy way to do it!
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