on / off delay timer

[RF Lighting]

Hello all.

I've got a system in a house that controls two immersion heaters.

It is just a relay energised from the off peak supply that switches either the peak or the off peak contactors contactors on.

The problem we have is when the OP supply switches on or off, the set of contactors coming on are switching in quicker than the ones going off are dropping out so momentaarily both the peak and OP supplies are connected together causing RCDs to trip.

I'm just after a simple device that could be fitted to stop this happening.

The control relay is just an 8 pin plug in jobbie.

I was wondering if theres something like a relay that moves slowly so that the first contactors can drop out before the others are energised.

Here's a sketch

thanks

 

[PrenticeBoyofDerry]

edit just looked at diagram!

 

[ban-all-sheds]

Aren't you 74 days late?

 

[matt1e]

There certianly is RF, you can even get mechanical ones that will mount directly on a contactor as an auxillary contactor (used on some star delta motor start circuts for example)

will post some links or a drawing tomorrow ( this) eve when I get time if you get stuck beforehand

Matt

 

[Bolty]

Looking at the diagram, can you connect the coil contacts through a normally closed contact from the opposite contactor?. eg the coil for p1 is fed from a n/c contact from op1 and coil for op1 is fed from a n/c contact from p1. (and the same for p2 and op2).

This way only when then the peak contactor has dropped out, will the off peak contactor energise and vice versa.

If this isnt enough, you do get mechanical interlocks that fit between contactors to ensure both cant be energised together.

Bolty.

 

[bernardgreen]

You might be making this more complicated than is necessary by switching the heaters from one supply ( all day ) to another supply ( off peak )

If you only have one meter that has two different rates for day and off peak then you can connect the immersion heaters to the all day supply as the meter will charge the correct rate depending on whether it is off peak or not.

If you have two separate meters then you need two "delay on operate" contactors are all that would be needed. ( it might be cheaper to use two normal contactors for the load and two low current delay on operate relays to delay the contactors.

 

[Lectrician]

Concur with bernard - If it is a simple dual rate meter, the heaters can be fed from the peak supply, signalled from the off peak.

With three phase off peak, it is common to have a pair of volt free contacts from the DNO to be used to switch what you want (either a contactor for a whole TP DB, or a couple contactors for a few circuits).

In your setup you should be using interlocked contactors, both mechanically and electrically - mechanical interlocks sit between the contactors and bolt them both together, electrical interlocks simply utilise NC contacts on both contactors, the coil from one contactor passing through the NC of the other and visa-versa. The interlock will prevent one contactor operating until the other has opened. Used regularly on reversing circuits on TP circuits such as roller doors etc.

 

[ban-all-sheds]

I'm annoyed here - I started a reply last night after PBoD's suggesting supplying the coil of each contactor via the NC contacts of its mate, then looked again at the diagram and decided it was a wind-up because for each position of the control relay there's a supply to each heater so the whole thing was pointless. Now I look again and think using my daytime brain I realise I was being dim.

Could the whole shooting match not be simply replaced with a single 4-pole contactor with it's coil energised by the off-peak supply?

 

[securespark]

My noggin hurts.

 

[ericmark]

This is very like standard star/delta and one can get some delay as the magnetic flux drops. Either use mechanically linked contactors and/or use a normally closed contact on each contactor to energise the other.

 

[RF Lighting]

A four pole contactor wouldn't work as you need to switch both the line and neutral to prevent and borrowed or commoned neutrals from different supplies. I don't thing my diagram is the clearest. Each contactor (P1, P2, OP1, OP2) have a 20A DP supply to them which I didn't show to try and avoid confusion! The control is shown going into the contactors, and then the DP output for how they are commoned on the output.

Unfortunately the setup is two supplies from the cutout through two meters and then the OP also goes through a sangamo time switch.

It's wired exactly the same as a star delta starter would be, but back to front so you have 2 inputs and one output, and using the OP to switch between contactors rather than a delay timer.

Unfortunately the contactors I used are hager modular contactors, so there are no auxiliary contacts on them.

I've been having a look around and have found these.

They have a variable delay of 0.1 to 10 seconds. I was thinking one of these set to about 1 second to feed each pair of contactor coils as below ought to cure it.

Does that seem feasible?

I think I've probably over complicated the whole setup a bit now?

 

[Lectrician]

It's feasable, but an interlock would be preffered, but if you have the contactors already....

I would set the time higher, even on max, 10 seconds is not exactly going to cause issues with the heating!

 

[flameport]

causing RCDs to trip.

Do these circuits actually need RCDs?
Get rid of them if not, problem solved.

 

[OwainDIYer]

why can't you use an ordinary immersion heater controller?

http://www.tlc-direct.co.uk/Products/HOELEC7.html

controls two immersion heaters and can be programmed for a daily peak boost.

 

[RF Lighting]

causing RCDs to trip.

Do these circuits actually need RCDs?
Get rid of them if not, problem solved.

Unfortunately yes, otherwise they would have been long gone. I still don't like putting heating elements on RCD

why can't you use an ordinary immersion heater controller?

http://www.tlc-direct.co.uk/Products/HOELEC7.html

controls two immersion heaters and can be programmed for a daily peak boost.

Where's the fun in that?