Hot answers tagged

75

simulate this circuit – Schematic created using CircuitLab Figure 1 and 2. showing the danger of earthing through neutral. In the first example the Lunatic 'Lectrician has 'grounded' the lamp case by connecting it to the neutral wire. All appears OK although the customer notices a slight tingle when she touches the lamp when it is switched on. This ...


56

You can't float everything without an isolation transformer - the neutral will be connected to ground at the entry into the building. For safety any class I equipment does need to remain grounded, in my lab I have an isolation transformer only for the equipment under test, the scope and power supplies still have the case grounds, and are tolerant of the ...


51

There are four reasons for grounding the neutral. 1. Grounding neutral provides a common reference for all things plugged into the power system. That makes connections between devices safe(r). 2. Without a ground, static electricity will build up to the point where arcing will occur in the switchgear causing significant loss in transmitted power, ...


37

Oscilloscopes usually require significant power and are physically big. Having a chassis that size, which would include exposed ground on the BNC connectors and the probe ground clips, floating would be dangerous. If you have to look at waveforms in wall-powered equipment, it is generally much better to put the isolation transformer on that equipment ...


31

WARNING: Means of being less likely to die or to kill others is discussed below. Reading may be considered useful.. Consider this "informed opinion" rather than holy writ. In an oscilloscope you want the best result you can get for the money and it's far easier to build a single ended amplifier rather than a differential one when you want ruler flat gain ...


21

Problems: First, currents don't "come from" the positive terminal. That's a very common misconception, called the "sequential fallacy" in grade-school electricity textbooks. The basic problem is that wires are not like empty pipes. And, the power supply doesn't fill them up. Instead, wires are already pre-filled with charge, so that currents always ...


21

If your plastic pellets are getting charged, you can flood the inside of the silo with ionized air using a commercial generator. These are not uncommon in the plastics industry.


21

There's more than one way to skin this cat, even to this very day While there is a global standard for mains earthing systems, IEC 60364 to be precise, it does not set out a single means of mains earthing. Instead, it defines three basic ways of performing the mains earthing function, and divides one up further into three subcategories: Terra-Terra (TT) ...


18

A voltage source has both negative and positive terminals, and produces a voltage (or potential difference) between those terminals. In The Beginning, the early scientists studying electricity had no means of determining what, if anything, comprised an electric current, so they somewhat arbitrarily declared that current was a flow of positive charge,flowing ...


16

What you're talking about is an isolated system. I have an extended treatise on it here. In an isolated system, "the first ground fault is free" (and becomes the neutral-ground bond). This is the idea you are promoting. The problem is the second one. Unless you have maintenance staff actively doing isolation testing and chasing down and eliminating that ...


14

Firstly, note that in some parts of the world, neutral and ground are NOT connected at the house, but back at the substation (transformer) so there may be a few volts on neutral in those systems. But to the question : Consider if you did just connect chassis to neutral. Then what happens if part of the house wiring fails open circuit? 1) Live (Hot) fails ....


14

On an IT-network, where both lines on the socket are live, the GFCI wouldn't work on a single fault. Which has benefits in in some high continuity systems (eg: operation rooms), a single fault doesn't turn everything off. But you will need to actively monitor for single faults using insulation monitoring. Instead, we ground neutral so that even on a single ...


13

While usually neutral wire and earth wire are at the same potential, and can therefore be effectively swapped from a purely electrical point of view, the way they are connected makes them very, very different. A domestic electrical system should always have two protection devices: the RCD, aka residual current device a circuit breaker The RCD is connected ...


11

What is the right way to float my home lab? If you want to make measurements that are isolated from ground, the only way to do this is with an isolation transformer if your scope is not isolated. There are very few reasons to do this, a high voltage setup would be one reason. Some AC measurements would be another. Differential probes are best. Do I ...


10

There is a problem. Generally the neutral and earth are connected at source. On fixed installations (e.g., your house) this may be the local transformer or at your meter-box, depending on local regulations. On your coach the generator / alternator most likely has its neutral connected to the chassis. The advantage of this is that we no longer need to fuse ...


9

A big reason that scopes are earth-grounded is that they generally have two or more channels which share a common ground reference. If a scope isn't earth grounded and one of the probe's ground inputs is connected to an ungrounded exposed metal chassis while another ground input is connected to AC120, the first probe could electrify aforementioned chassis ...


9

It does work that way. The problem is that you are comparing an ideal concept of Earth (e.g. one uniform level of potential energy) with a realistic concept of a floor... and that doesn't work. The problem is that the floor is highly resistive. The ground is too if you don't bury a large enough conductor deep enough (e.g. deep enough to access the ...


9

In a nutshell Electricity is not supposed to flow through ground stakes in normal conditions. It doesn't mean its resistance is high, it's actually surprisingly small. That branch of the circuit is simply not closed normally. In details A ground is a reference point. You could litterally take any net in your circuit which is supposed to stay at a steady ...


9

Can static shocks occur from grounded and ungrounded metals? Yes. A large ungrounded object still has capacitance. If there is a higher density of electrons on you than on the object, the electrons on you will feel repulsion from the other electrons on you and want to jump to the object where they will have more room to spread out. (Same goes in reverse if ...


9

That is actually an interesting question. The truth is... it is complicated. Consider the following three connection systems. simulate this circuit – Schematic created using CircuitLab The first is your typical appliance with a three pin plug and a grounded case. If there is a short from the live line to the case the line is shorted out and ...


8

Vladamir has the important answer: we treat the two conductors differently, even though they are nominally at the same voltage. He gives the example of a RCD, and the GFCIs you see in modern US construction operate very similarly. The expectation is that the current flow of the hot wire and neutral wire will be equal in magnitude. If they are not, the RCD/...


8

As Neil has pointed out, the big picture is that you are part of a big electricity network, and if it wasn't grounded somewhere, the whole damn thing would float high - perhaps to $lightning volts. Your second question "Wouldn't it be safer to just float it", becomes a very interesting question when you have a local, unconnected solar power system. The ...


8

There is no right way to float your home lab. 1) No. Right way is not to float equipment that must be grounded. 2) No. Don't float any of them. Equipment that have grounded plugs NEED to be grounded for a reason. 3) No. Because again, equipment with ground pins need to be grounded! Having all lab equipment and the device being examined being connected to ...


7

If by plan B you mean to add a resistor to the mains ground, then no plan B is not an option. If you add a resistor in series you will have two problems: 1) It affects the saftey of the earth mains, if you size the resistor wrong in the event of a fault you could have an issue with the resistor blowing out or creating common mode voltage in the case 2)...


7

Reverse the polarity of one of the speakers. It sounds as though you have one of them reversed and so they are out of phase. Vocals and bass are usually centered in the stereo panorama and when they are played out of phase on left and right channels they tend to cancel out in the centre of the stereo image. The reason for this is that one speaker is pushing ...


7

If everything you do is DC, all you need is a DMM. Likely what you meant is that it is low voltage but DC to high frequency. Earth ground is advantageous two good reasons; 1) safety the line filter noise currents to the metal frame will go thru you if not earth grounded but your body has stray capacitance to earth line transients 2) performance EMI ...


6

I am not sure about UPSs, but a diesel generator is built to generate a voltage between its two (four) output terminals. Whether these terminals voltage with respect to earth is high or low is not defined, and can't be. The generator per se is not connected to earth, so its neutral terminal might be some volts above (or below) earth as well as some (tens of) ...


6

The reason you don't get a powerful current is that the conductivity of air is so low. Since the conductivity of the copper wire is so much higher, the electric field itself is modified. Basically, charges do move around in the copper wire, which means that you have to consider the entire wire to be at the same potential. This causes the nominally vertical ...


6

The following is with regards to how waterproof enclosures are installed in industrial environments, where IP56 is the usual minimum standard (if not IP66 or IP68.) With regards to waterproof grommets for wires - the correct name for these is a 'cable gland' and they are available from IP56 up to IP68 ratings. The metal versions last better than the plastic ...


6

Answering the question in the edit: If the bulb is connected this way: If the customer touches the lamp when it is switched on, I do not thing she >will notice a slight tingle because the return path is the ground. If the neutral wire fell off, the metal case is not live. If the ground >wire fell off, the bulb will just turn ...


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