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.

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

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.

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

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

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….

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.