Controlling Motor Surge

[shedbang]

Hi all

I am the proud owner of a mid-size scissor lift, with which I can lift up cars. I've owned this for around a year, but just moved house and after a Herculean feat of removal, the ramp came with me.

For the new house, I had a sparky re-wire the garage (and half the house!) to accommodate it. He ran 10mm cable from a vacant 32A fuseway in the (new) house CU, to a small CU in the garage with a dedicated 32A MCB feeding a blue commando socket into which said lift is plugged. The RCD is at the house end meaning that in the garage we just have a plain switch with the MCB's.

In the old house the RCD was at the garage end, and about one time in five, the motor surge would pop the MCB, the RCD or both. No problem - I'd reset and try again.

In the new house, the thing does it about 3 in 5 times, which given the walk between garage and house is going to get annoying with the continual concern that a car will be left suspended in mid air. When it goes it blows one, two or all of the garage MCB, the house MCB and the house RCD.

The motor is rated at 14.7A and 2.2Kw so I think that the circuitry should be up to scratch but I guess this means the motor must be pulling a lot more than 32A at start up.

I'm not sure whether to call the sparky or the ramp manufacturer (it is due a service). I've looked at soft start protection, but this looks like it would need me to modify the ramp circuitry, which I'm not confident in doing (i.e. I'd have to phone the manufacturer). If there was a kind of reverse surge protector that could be fitted in line, that would be just the job, but such a thing simply doesn't seem to exist.

Any thoughts / help greatly appreciated.

 

[plugwash]

I'd say speak to the ramp manuafacturer first. Even if ultimately the soloution is to modify the wiring and/or protective devices in your house you should still get thier advice.

Can you give more details on this 10mm cable? is it T&E? SWA? If T&E is it a concealed cable?

Are the breakers type B or type C?

 

[RF Lighting]

How long is the cable run from the house to the garage? Is it definately 10.0mm² SWA? What is the earthing supply type at the house? Do you have any water, gas or central heating pipes in the garage? What is the rating of the car ramp? There will be a metal plate with this information stamped onto it somewhere on the ramp.

If you are not sure then a couple of photos would be very helpful. Take one of your incomming cutout / meter / fusebox, one of the ramp rating plate and one of the garage consumer unit.

Personally it just sounds like a poorly designed installation especially knowing you've got a car lift going in there, but I suppose a domestic installer might not know any better, apart from the lack of discrimination you have.

Ideally you want to split your incommimg mains at the meter tails into a henley block and re-feed the house CU and a new switch fuse fitted with either a BS88 or a BS1361 cartridge fuse, NOT a circuit breaker.

From the switch fuse feed the SWA to the garage. The garage end is going to need a bit of re-jigging. You probably don't need RCD protection to the ramp, and depending how the lights are wired you might get away with out RCD there too. I'd be looking at putting the sockets on there own RCBO.

You don't need soft start, and I doubt there's anything wrong with the ramp, you just need a proper motor circuit installing. It shouldn't trip ever unless there is an actual fault which I don't think there is.

 

[shedbang]

RF - your reply looks about right. To answer (I think) all the questions:-

- Supply is T&E through the house, switching over to SWA outside. It's all definitely 10mm.
- The run is probably - guessing here - 30-40m.
- Ramp motor is rated at 14.7A and 2.2kw. Don't think there's another metal plate on the housing aside from the one with that written on it.
- Earthing supply is PME.
- There is an old lead water standpipe in the garage. Comes straight out the ground. Not that far from the electrics. Guessing lack of RCD would require that to be bonded somehow.

The supply to the garage feeds a simple ring final and the lights as well as the ramp.

The modification you suggest would be fairly easy I guess for him to correct? What would you put in the garage CU for the ramp? Type C MCB?

Thanks. Very helpful.

 

[RF Lighting]

Just done some calculations on guessed values and you're borderline for a 32A type C circuit breaker in the garage. You would need to have the Zs reading confirmed in the garage before this could be installed.

If the system is suitable for a C32 then I think that (along with the other recommendations) should cure your problems. If not we might have to look at cartridge fuses or similar.

Now there are a couple of issues. As you have a metal water pipe in the garage then you need to extend the equipotential zone from the house to the garage, or make the garage supply TT.

I would prefer to see the bonding extended, but this requires a ≥10.0mm² earthing conductor from the house MET to the garage.

A 10.0mm² 2 or 3 core SWA is big enough for this but the 10.0mm² T&E isn't, so would need an additional earth running in from the CU to the SWA joint. (This would also reduce the Zs at the garage and give you a better chance of being able to install a type C circuit breaker)

If this is not feasible then the garage supply will need to be converted to TT which will involve isolating the earth from the house at the garage end, and the installation of earth rod(s) at the garage and every circuit would require RCD protection.

Is the 10.0mm² T&E cable a any point concealed within a wall or partition at a depth of less than 50mm from the wall surface?

It's not mega works to put this right as most of the cabling is in, it's more just a case of rejigging it all to be a bit more suited to your requirements.

 

[shedbang]

RF - again, thanks for your help.

Routing the T&E was the expensive part of the job, so routing some fat earthing alongside it is not something I'd like to contemplate. Making a TT earth in the garage on the other hand, would, in theory, be relatively easy.

Tempted to hack out the lead pipe, but (get this) we have no idea where the stop cock is for it. Suspicion is that it is plumbed direct (it's a pretty ancient pipe!).

When you say every circuit in the garage would require RCD protection, does that include the ramp (if so, we're back to square one).

The T&E is clipped direct and visible for much of its run to the garage (we have it running along the inside of the various roofs). Where it is not visible it is either in boxed off sections (>50mm) or under floors. However, there is one section behind plaster in a safe zone, with metal capping over it. I think this measn we're okay on RCD protection. If not, I'm in trouble.

As mentioned, the pricey bit was pulling up floors and boxes to run the cable and the cost of the cable itself. It's that bit I want to avoid re-doing / modifying!!

Once again - appreciate your time on this. As the Sparky didn't manage to get it right last time, I want to make sure I know EXACTLY what I'm asking for this time around!

 

[ban-all-sheds]

I suppose a domestic installer might not know any better, apart from the lack of discrimination you have.

He installed an IEC 60309 socket - he knew there was going to be more than a hand tool or mower etc plugged in. He should have asked about the characteristics of the load. No excuses.

He knew it was a PME supply and that there were extraneous-conductive-parts in the garage. He should not have exported the PME earth like that. No excuses.

Lack of discrimination? Poor design. No excuses.

RCD for the outbuilding final circuits remote from the outbuilding? Poor design. No excuses.


He should not be working as an electrician - he is incompetent.

 

[bernardgreen]

It is likely that the 10.0mm² is NOT adequate for the motor's current during start up.

It will be once the motor is running at speed and therefor taking its rated current.

But while the motor is accelerating up to speed it will be taking a much higher current with a correspondingly higher voltage drop along the supply cable. The higher voltage drop means the motor will take longer to reach full speed and this longer period may be longer than a slow operating MCB will allow before it operates on the overload of starting current.

 

[shedbang]

He should not be working as an electrician - he is incompetent.

Whilst your sweeping judgement may well be true (accepting that you have not seen the installation and are accepting the description of an amateur) it is not especially helpful.

So help me out. Exercise your not inconsiderable brain in assisting me explain in plain English what my 'clearly' incompetent electrician needs to do to fix this.

I am a punter with no garage power supply. I have a PME earth. I have a 100A fuse protecting the house. The garage is (max) 50m from the fuse box through the house, and across some driveway. It has a 2 ft lead standpipe in it which has no continuity with the house pipe work. I wish to run a car lift with a 14.7A 2.2kw motor at its core. This lift is fitted with a 32A commando plug. I'd like say 5 double sockets, mostly used for battery chargers etc. but with a maximim load of a compressor, fitted with a normal 13A plug. I'd like 2 fluorescent lights and 2 bare bulbs. The garage is around 35m2. I have a modern 17th ed CU in the house.

Based on the information above, what should I be looking for in circuit design to fulfil my requirements?

 

[ban-all-sheds]

Meter tails split, switchfuse supplying garage CU via 16mm² SWA.ª

Garage pipe bonded.

C32 for the lift.

RCBO(s) for normal sockets.

MCB(s) for lights. Fluorescents should be high frequency, at least one to be a non-maintained emergency luminaire.

ª Disclaimer - I haven't actually done the calculations to see if that's needed - just going by RF's observation that 10mm² is borderline for a Type C breaker, and bernards thought about startup current.

http://www.tlc-direct.co.uk/Book/5.3.1.htm

//www.diynot.com/wiki/electrics:mcb2

 

[bernardgreen]

You need to find out the stall current of the motor and the acceleration time of the motor. That will decide the type of overcurrent protection required in the garage for the circuit supplying the motor. Having that protection in the house creates a risk of you being plunged into darkness when the lift is started.

You need a supply from the house with a slow acting overcurrent device that will tolerate the worse case motor start up without tripping. The cable from house to garage should be capable of surviving any overload that slow acting overcurrent device will permit should there be a dead short due to damage in the garage. Hence the cable may need to be of considerably larger cross section than is required for normal rated running current. At the garage you would then separate and fuse down for the motor, the sockets and the lights.

Better would be two supplies from the house, one as above but only for the motor and a separate supply for lights and sockets.

You also need to consider notifying the DNO about the motor as most DNOs do require notification of any motors or other inductive loads greater than 1 kW.

 

[shedbang]

Guys thanks

This is a royal pain, but the 'proper' solution is I would imagine to run a new feed to the motor in SWA very much as Ban suggests. Very difficult in grade 2 listed house.

I can't immediately see references to stall current and acceleration time. It's a pretty big motor with 'two capacity capacitor motor' written on it. Without a detailed spec sheet (I don't have) I doubt I'd find out.

Following your comment about the DNO Bernard I took a look at my compressor. That's also a 2.2kw job but runs off a 13A plug and has never caused a problem. Are you really supposed to notify these devices? I did mention it to the DNO guy who uprated our fuse. He gave me the usual quizzical look that people do when you tell them you have a car ramp in the garage, but nothing else.

I can't help wondering whether the ramp would benefit from some soft start protection and that would solve the problem. I know very little about electronic components, but looking at the ramp internals, I can't see any obvious soft start kit. Before I lay down many more pounds on re-wiring (again) I'll call the manufacturers and see what they say.

 

[bernardgreen]

Are you really supposed to notify these devices?

I had to list any motors or transformer connected loads greater than 1 kW on the application for a new supply to my cottage. Even a medium size air conditioning unit was considered as notifiable. If your lift when it starts causes a voltage dip in the network that affects neighbours then the DNO will be looking for problem. If they are aware of you motors then then they need not examine their network for a non-existant fault.

I can't help wondering whether the ramp would benefit from some soft start protection and that would solve the problem.

Soft start can prolong motor life by removing / reducing the over current and mechanical shock wear and tear on the motor.

Very difficult in grade 2 listed house.

I have the same situation, put a case forward and trade something the conservation officer wants you to do ( and you don't ) in exchange for permission to run extra cables. After all you do not want to take any risks with the electrics that might end up in the loss of the building that is listed as being of historic value.

 

[RF Lighting]

The ideal soloution would be a brand new length of 16mm² SWA direct from a new 80A switch fuse to he garage, but we are stuck with what ypou've got, so now need to come up with a soloution to work around the supply you've already got.

I would have thought that a 10mm² supply should be more than able to cope with the volts drop from a 2.2kW motor starting.

I know we are going back to basics here but what type are the existing MCBs in the house and the garage?

Is any of the 10.0mm T&E cable buried within the walls of your house? If not then we can sack off the RCD at the house and keep all RCD protection local to the garage.

It's looking like TT is going to be the way to go for the garage. If we can get rid of the house RCD you would still need 30mA protection for your socket outlets, but you could use a 100mA time delayed RCD for the ramp which is going to be much less prone to nusiance tripping.
The lights don't actually need 30mA protection, but a 30mA device is the most common, so will be the cheapest soloution.

 

[ericmark]

I did have some problems with a pair of pumps tripping out a single phase supply and as you have rightly said swapping to a three phase motor and inverter control did cure the problem.

However that is an expensive option as not only do you need the electronic start but also change in the motor.

The problem with starting a pump, and I would guess the ramp is hydraulic? is as it starts the load increases so unless started within a set time is can stall because of the hydraulic pressure. So playing with capacitor sizes does not really work where the motor is loaded at start rather than free running at start and using a clutch once running.

Since you don't have a three phase supply can't really use a soft start it would need to be a full inverter.

Cable type is also a problem. With cables like SWA and mineral insulated having the RCD on the destination of the cable is not a problem but with twin and earth often it does need to be at origin.

One trick which might work as far as the MCB goes is to have a grinder or other motor with large flywheel running first. I used the shears sharpening tool at one farm I worked at. The same problem with generators many will not run large motors due to the start load.

The Ward Leonard and Inverter both will work but both are very expensive so it's not really a question as to how but more a question of most cost effective method. May be a generator will work but I would hire one first and try it.

The problem is money. If you were running a garage and your electrician told you it will cost £750 and it will take 2 days one way or the other is to get a supply direct to garage which may take 2 months to get DNO to agree to you would not argue you would pay the £750 and thank me for finding a way around the problem as you would be losing business while you waited. I would assume your not in a hurry and you want it on the cheap?

So what is needed is a costing exercise for all the methods then select the best one for you.