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What RCD NVR for a workshop?

tecchy leccy query

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Ady103/07/2021 09:21:46
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I noticed some chat in this thread and there are some threads about but it's all a bit higgilty piggilty for such an important and useful subject

I have an RCD box in my garage which cost almost 200 quid which annoyed the heck out of me when it was installed

But it's now essential kit for linking everything up because of its advantages and more importantly I can use my lathe/shaper AND welder while connected up to it thereby getting its advantages

So I have 2 questions

1 Whats the difference between a RCD and a NVR

2 Can anyone recommend a not too expensive plug in unit which will do workshop equipment to 1.5Kw and welders to... dont know, 150 to 200Amps?

Single phase of course

Even if I don't use it in the garage it would still be a handy portable asset

cheers

Edited By Ady1 on 03/07/2021 09:28:19

Emgee03/07/2021 09:40:28
2147 forum posts
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Ady

The RCD you refer to is also known as a residual current circuit breaker RCCB, if for any reason the current in the live conductor is not the same as the neutral conductor the device senses the unbalanced load and trips the device off.

Some plug-in RCD units have NVR, this means if the power fails to the socket the unit trips to off, you need to reset to use again.

Some plug-in RCD units don't have this NVR feature so would be powered up again when power is restored in the above example.

If you have an Installation protected by a 30ma tripping current RCCB on the incoming supply there is no need for further devices unless you want the NVR option.

Emgee

Edited By Emgee on 03/07/2021 09:41:40

Anthony Knights03/07/2021 09:53:23
555 forum posts
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An RCD (Residual Current Device) monitors the current in the live and neutral supplies and will open the circuit if there is an imbalance (ie there is a leak to earth). These are usually installed in the distribution boards, but small adapters are available for use with portable power tools.

A NVR (No Volt Release) is normally fitted as the on/off control for a machine tool ( lathe, pillar drill, etc). In the event of a power failure it will switch to the OFF position so the machine does not start un-expectedly when power is restored.

1.5kwatts at 230 volts is about 6.5 amps, so a standard British 13 amp plug should stand 3kwatts. A load of 100 amps is liable to blow the input fuse to your house.

Ady103/07/2021 10:18:45
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I always run my 120A welder through a heavy duty garden 30M reel and it's been fine both pre and post RCD

If you have an Installation protected by a 30ma tripping current RCCB on the incoming supply there is no need for further devices unless you want the NVR option.

So is that what we're looking for on a plug-in RCD unit? 30ma

Nicholas Farr03/07/2021 10:41:11
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Hi Ady1, 30ma tripping is the standard to help prevent most people being electrocuted by being in contact with a live wire as well as detecting an imbalance due to any fault, the NVR part just drops out if one or both the supply wires are disconnected, i.e. if there is a power cut or the socket you plug your unit into is switched off. In both cases the unit will have to be reset.

Regards Nick.

Edited By Nicholas Farr on 03/07/2021 10:45:36

Ady103/07/2021 10:56:54
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Thanks chaps

Bob Worsley03/07/2021 11:02:34
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Slight complication if you use inverters to run three phase motors from single phase, you must use the Type B ( as far as I can remember) type of RCD.

The RCD works by measuring the imbalance in the current in the live and neutral wires, the problem comes when there is a DC current also in the wires, this saturates the inductor and it will never trip. Hence no safety. Some other devices also put a DC load on the mains, but it is the inverter that is the big problem with model engineers. There is discussion about the old practice of using half wave rectification in TV sets, but they didn't have RCDs then, just the ELCB, earth leakage circuit breaker, no where near as good.

Never ever run a high current load through an extension lead just for the hell of it! If you forget to FULLY unwind it then you will get one hell of a surprise as it explodes. Always fully unwind extension leads for loads over just a 100W or so. The reason? The wound extension lead is a transformer or choke winding, and forcing current through it causes it to heat due to the magnetisation and hysteresis losses as in any inductor. Added to this you are adding in some extra resistance, but more importantly lots of synchronous impedance which will effectively stop the welder working as it should. The welder uses a variable choke to introduce synchronous impedance into the output winding to limit the current.

Nicholas Farr03/07/2021 11:19:04
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Hi Bob, re, unwinding extension leads. I said to a workmate many years ago about a 110v extension lead he was using, still mostly wound up, that he shouldn't be using it like that, he said he was only using a vacuum cleaner and a lead light and it has been OK, but no sooner he got his words out, the extension lead was billowing out smoke. It was just as well I happened along as he was in a machine he couldn't get out of in too much of a hurry, so I unplugged it for him, needless to say the extension lead was totally shot, he didn't use one like that again.

Regards Nick.

Edited By Nicholas Farr on 03/07/2021 11:22:52

Dave Halford03/07/2021 11:38:43
1669 forum posts
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Posted by Nicholas Farr on 03/07/2021 11:19:04:

Hi Bob, re, unwinding extension leads. I said to a workmate many years ago about a 110v extension lead he was using, still mostly wound up, that he shouldn't be using it like that, he said he was only using a vacuum cleaner and a lead light and it has been OK, but no sooner he got his words out, the extension lead was billowing out smoke. It was just as well I happened along as he was in a machine he couldn't get out of in too much of a hurry, so I unplugged it for him, needless to say the extension lead was totally shot, he didn't use one like that again.

Regards Nick.

Edited By Nicholas Farr on 03/07/2021 11:22:52

Lent my daughter my Roverpoint (remember them) she didn't unwind it and the cable all melted together into a solid mass.

SillyOldDuffer03/07/2021 11:42:50
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RCD and NVR are different safety devices.

No Volt Release switches trip OFF when the power input fails, and have to be reset manually. Their purpose is to stop machines starting on their own after a power-cut. Cutting tools are particularly dangerous if this happens, both to the operator who might have his hand in the cutting path whilst investigating the stoppage, and to the work if the machine restarts in an unknown position after stalling. Not all machines need an NVR. Many, like Immersion Heaters or TVs, don't need an NVR because they restart safely after power cuts.

A Residual Current Device detects Earth faults, and are sensitive enough to trip when a human has connected himself between live and earth and is being electrocuted. A multitude of faults cause earth faults: age damaged insulation, accidentally cut wires, wires falling out of loose terminal blocks, switches falling apart, maintenance mistakes, component failures and much else. RCDs and Fuses are similar except fuses take a lot longer to blow. They often take so long to blow that a fatal shock is delivered, where an RCD wouldn't. Other advantages, RCDs can be reset without fiddling with new fuse cartridges or fuse wire - many people fit the wrong size - and the RCD doesn't depend on the house having a good earth. Many domestic electrical earths are poor; a spike bashed into indifferent ground, or a strap on an old water pipe that may have been connected to a plastic main since the house was built, or even strap connections to an ancient rising gas main. Domestic earths worsen over time due to corrosion and earth wires within the building are subject to damage.

NVRs are highly recommended for machine tools and it is illegal to sell new ones without them.

RCDs are safer and more convenient than fuses. An RCD Consumer Unit is safer than a fuse box, and it's worth fitting an individual RCD to mains electric power tools especially in the garden. Most modern reel-type extension leads have one built-in, or they can be bought as a plug-in adaptor.

There is a disadvantage when RCDs protect equipment fitted with an electronic drive, like a VFD. Electronic drives are fitted with a filter designed to stop electronic muck from getting in or out of the equipment. The filter works by shorting the muck to earth causing a current balance at the RCD, possibly big enough to trip it. In an average home almost every device has some form of filter, and these can add up enough to trip an RCD. I suspect plugging in a big motor with a hefty VFD in a home workshop is often the straw that breaks the camel's back! It's not necessarily the VFD at fault. When a workshop trips the RCD, it's worth unplugging everything else in the house to see if that eliminates the problem. If it does, plug stuff back in one at a time in hope of isolating the troublemaker. It may be that one particular device has a leaky filter and is already bringing the RCD close to tripping point. Someone on the forum, I forget who, identified their microwave oven as the real problem.

Dave

 

Edited By SillyOldDuffer on 03/07/2021 11:43:16

Steve Skelton 103/07/2021 17:27:10
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SOD Sorry Dave, I have to challenge you on a couple of points re the above:

1 RCD's and Fuses are not similar they do completely different things. An RCD detects leakage current that could potentially be going through a human. Fuses on the other hand are there to protect the wiring and prevent overload and potential fires. An RCD will not detect overload if there is no leakage current whereas a fuse will blow at a predetermined level. RCD's detect the difference in milliamps between the phase and neuitral conductors, fuses in household circuits operate generally at between 6 amps and 45 amps

2 When you say an RCD Consumer unit is safer than a fusebox, that again is not strictly true for the same reason as 1. If you are saying that MCB's are safer than single-use fuses again that is not so, the only advantage of an MCB is that they are resettable. An MCB is a mechanical device that can (and frequently do) fail. A correctly specified fuse is the safest means of detecting and disconnecting a supply in the case ofcurrent overload.

Steve

old mart03/07/2021 17:35:01
3313 forum posts
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We had a volunteer at the museum who ran a 2Kw electric heater through a partially unwound extension reel and the inner 12 feet had to be removed as it had started to melt. He had been an aircraft electrician, so he should have known better.

I was concerned with the RCD's when setting up the 0.75 Kw VFD, but it has not tripped so far.

Edited By old mart on 03/07/2021 17:37:48

SillyOldDuffer03/07/2021 20:37:19
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Posted by Steve Skelton 1 on 03/07/2021 17:27:10:

SOD Sorry Dave, I have to challenge you on a couple of points re the above:

1 RCD's and Fuses are not similar they do completely different things. An RCD detects leakage current that could potentially be going through a human. Fuses on the other hand are there to protect the wiring and prevent overload and potential fires. An RCD will not detect overload if there is no leakage current whereas a fuse will blow at a predetermined level. RCD's detect the difference in milliamps between the phase and neuitral conductors, fuses in household circuits operate generally at between 6 amps and 45 amps

2 When you say an RCD Consumer unit is safer than a fusebox, that again is not strictly true for the same reason as 1. If you are saying that MCB's are safer than single-use fuses again that is not so, the only advantage of an MCB is that they are resettable. An MCB is a mechanical device that can (and frequently do) fail. A correctly specified fuse is the safest means of detecting and disconnecting a supply in the case ofcurrent overload.

Steve

No need to be sorry Steve, you make excellent points. Always happy to corrected: better the forum gets the right information than my ego be mollycoddled! Accuracy always gets my vote.

Ta,

Dave

Steve Skelton 103/07/2021 22:11:44
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Dave, thank you for such a positive attitude, I did think twice before posting as often you see people getting hot under the collar when someone questions something that they posted.

I personally would never fit a fuse rather an mcb but they are much more reliable, as my NICEIC assessor always kept impressing on me during my annual assessments.

Cheers Steve

Zan04/07/2021 00:10:55
280 forum posts
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My workshop electrics are fed as a separate ring main from the house consumer unit via a 30 amp trip

by the w shop door is a 24 amp large circuit breaker with built in trip. This means any shop problems do not trip the main house unit. I use this as a main power switch and turn all off when exiting, so all the led machine lights and other stuff have no chance of running without me there. It has worked brilliantly for 18 years.

i fitted a new inverter to my Bridgeport . Now when switching on this machine, it trips my shop breaker randomly, about 1:5 ratio, it triggers the house every 3-4 months. It has nothing to do with the electrics in the machine cabinet, as I have systematically rewired these so that nothing was working and the inverter was directly wired to the mains plug, using the socket to power up. Motor start is via the remote digital controls. Still had occasional trips even with this.
any ideas? Could this be a problem with the built in filter, as indicated above which a parameter change can disable….

Andrew Johnston04/07/2021 10:48:29
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Posted by Zan on 04/07/2021 00:10:55:

Could this be a problem with the built in filter, as indicated above which a parameter change can disable….

Unlikely. Contrary to the statement made by SoD many inverters (especially cheaper ones) do not have built-in EMC filters. Even if the inverter had an EMC filter it is unlikely that it could be bypassed via a parameter as it wouldn't be simple to disable the filter electrically.

It's not clear from the post if a breaker (over-current) or RCD function is being triggered?

Andrew

Steve Skelton 104/07/2021 11:00:03
118 forum posts
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Zan, you need to be a little more precise with the description of your electrical installation before anyone can advise you on potential problems.

Is your workshop within the confines of your house or is it a separate building? If it is separate then the earthling arrangements for your workshop may need to be separated from your house.

Ideally, your workshop should not be fed from the house consumer unit by a ring circuit. Does this circuit have any RCD protection at the origin?

I have never come across a domestic 24A circuit breaker. If so what type (tripping characteristics does it have)? Do you have RCD protection within the workshop?

Have you had the wiring tested to ensure all of the protective devices and wiring are appropriate?

From the sound it I think it would be wise to get an electrician involved who understands inverters - you may have the incorrect type of MCB fitted if there is a fairly large inrush current – if it is a B type circuit breaker you may need to change it to a C type.

Sorry, I can’t be more help - there are too many unknowns.

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