Voltage ratings of cables - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: March 09, 2023, 04:05:07 pm
content: What does it means when i see 450V/750 V or 600V/1000V on a cable. Can someone explain this to me. Many thanks in advance

author: Nusa
author_rank: Super Contributor
timestamp: March 09, 2023, 04:11:32 pm
content: Quote from: robsims on March 09, 2023, 04:05:07 pmThey're AC voltage ratings. The first number is phase to ground. The second number is phase to phase.

author: themadhippy
author_rank: Super Contributor
timestamp: March 09, 2023, 04:16:56 pm
content: The first is the voltage to earth,the second is the between conductors .

author: robsims
author_rank: Frequent Contributor
timestamp: March 09, 2023, 04:25:10 pm
content: Quote from: Nusa on March 09, 2023, 04:11:32 pmMany thanks, but what does it mean exactly? I think i'm dumb, i still can't see the relation. Please explain a bit more. Why phase to ground and phase to phase. Why is phase to ground lower than phase to phase.

author: themadhippy
author_rank: Super Contributor
timestamp: March 09, 2023, 04:29:48 pm
content: Take a 3 phase circuit with the phase 120 degrees apart,here in the uk the voltage between phase and ground is 240V and between phases its 415v

author: Brianf
author_rank: Regular Contributor
timestamp: March 09, 2023, 04:41:30 pm
content: Quote from: robsimsHow many other combinations can you think of?

author: robsims
author_rank: Frequent Contributor
timestamp: March 09, 2023, 04:43:52 pm
content: Quote from: themadhippy on March 09, 2023, 04:29:48 pmSo in my first case working voltage between phase and ground may not exceed 450 volts and voltage between phase to phase may not exceed 750 volts. Am i correct? And do these ratings have something to do with the insulation? Is the insulation of an earth cable thinner then that of a phase cable?

author: Nusa
author_rank: Super Contributor
timestamp: March 09, 2023, 04:47:38 pm
content: Quote from: themadhippy on March 09, 2023, 04:29:48 pmWhat he said, since 3 phase supplies have phases 120 degrees apart. Skipping the math, the relationship between the two numbers is 1.73.240 * 1.73 = 415. 600 * 1.73 = 1038 (rounded to 1000). 450 * 1.73 = 778 (rounded to 750).If you don't understand it, use the lower number to be safe.

author: Nusa
author_rank: Super Contributor
timestamp: March 09, 2023, 04:51:39 pm
content: Quote from: Brianf on March 09, 2023, 04:41:30 pmPlease don't change-quote me to saying things I didn't say.I answered the question. That is what that labeling means in the context of 3-phase power. Other combinations aren't relevant to that answer.

author: Brianf
author_rank: Regular Contributor
timestamp: March 09, 2023, 04:56:36 pm
content: Quote from: Nusa on March 09, 2023, 04:51:39 pmSorry. Corrected.

author: robsims
author_rank: Frequent Contributor
timestamp: March 09, 2023, 05:13:48 pm
content: Do these ratings have something to do with the insulation?

author: JohanH
author_rank: Frequent Contributor
timestamp: March 09, 2023, 05:26:39 pm
content: Yes and no. The cable doesn't magically change its insulation properties to adapt to a higher voltage when connected phase to phase instead of phase to ground. It's just that it has been tested, validated and certified for two different conditions that are requirement for these types of cables.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: March 09, 2023, 05:30:46 pm
content: When I see these numbers on wire, I always thought it means the insulation ratings for AC / DC voltages. Why phase to phase? How is this useful info? The insulation rating is critical.Please explain.

author: rstofer
author_rank: Super Contributor
timestamp: March 09, 2023, 05:48:21 pm
content: Quote from: Brianf on March 09, 2023, 04:41:30 pmCommon in the US: 120/208V 3 phase, 277/480V 3 phase (these two pairs are very common in industry), 2400-4160V 3 phase .All of these voltages pairs are square root of 3 apart. Because the primary is delta connected and the seconday wye connected (to allow grounding the midpoint) the sqrt(3) will show up somewhere.

author: themadhippy
author_rank: Super Contributor
timestamp: March 09, 2023, 05:50:35 pm
content: Quotebecause its not uncommon to have multiple single phase circuit cables in the same containment,but the circuits are on different phases

author: robsims
author_rank: Frequent Contributor
timestamp: March 09, 2023, 06:09:23 pm
content: Quote from: JohanH on March 09, 2023, 05:26:39 pmSorry, maybe i still don't fully understand it, i know i'm stupid, But ok lets say we have 4 wires with insulation in a cable. See attached picture. A blue wire, a brown wire, a black wire and a yellow/green earth wire. So the max voltage difference between the brown wire and the green/yellow earth wire is 450 volts and the the max voltage between the brown wire and the black wire is 750 volts. Is that correct?

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2023, 06:55:54 pm
content: Quote from: robsims on March 09, 2023, 05:13:48 pmNot really, certainly not at low voltages i.e. under 1kV or so. Even the thinnest of cable insulation will reliably hold-off a couple of kV. Various standards dictate the insulation must be a certain thickness and the core have a certain cross-sectional area. This is to ensure mechanical strength, so the the cable can still safely protect against shock and short circuits, even with minor damage. At higher voltage, yes insulation is a factor, but not at ordinary mains voltages.

author: Nusa
author_rank: Super Contributor
timestamp: March 10, 2023, 02:40:40 am
content: Note that yellow/green (most universal), green, and bare copper (in the US anyway) are colors used for safety ground, not to be confused with the neutral ground that we're usually referring to when we're talking single phase. There are color codes for lives and neutral too, but they're less universal. I don't know what standard your country follows.

author: jonpaul
author_rank: Super Contributor
timestamp: March 10, 2023, 05:32:47 pm
content: see professionnel Electricians forums and Mike Holt.comjon

author: robsims
author_rank: Frequent Contributor
timestamp: March 10, 2023, 09:17:21 pm
content: Just read an article of someone who was also confusedhttps://electronics.stackexchange.com/questions/640653/what-determines-the-voltage-rating-of-a-cable-and-why-do-some-cables-have-u-0Cable rating 450V/750V Uo is the first voltage U is the secondDoes the attached drawing correctly show the meaning of the two voltages. 450 between an insulated conductor and earth (if there is a metal sheet around the cable) and 750 volt between two insulated wires?

author: nightfire
author_rank: Frequent Contributor
timestamp: March 10, 2023, 10:38:06 pm
content: To better understand this, have a look at how 3-phase systems work. The active lines are 120 degrees spread to each other, therefor this system will have a certain voltage to the neutral point and a higher voltage between the active lines themselves- for ease of use both of them are printed on the cable.Wikipedia has some nice diagrams on the classic star/triangle setups:https://en.wikipedia.org/wiki/Three-phase_electric_power

author: robsims
author_rank: Frequent Contributor
timestamp: March 10, 2023, 11:35:52 pm
content: Quote from: nightfire on March 10, 2023, 10:38:06 pmI know how 3 phase systems work. I still don't seem to get this. I'll do further research

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2023, 11:39:04 pm
content: The voltage ratings on cables are not performance characteristics such as ohms/meter or capacitance, they are manufacturer's guarantees of what voltage is safe to apply under different circumstances.

author: robsims
author_rank: Frequent Contributor
timestamp: March 11, 2023, 12:25:06 am
content: Cable 450V/750 U0 = 450V U =750VU0 is the rated power frequency voltage between conductor and earth or metallic screen forwhich the cable is designed;U is the rated power frequency voltage between conductors for which the cable is designed;What is the metallic screen?

author: robsims
author_rank: Frequent Contributor
timestamp: March 11, 2023, 12:40:24 am
content: The rated voltage of a cable is the voltage on which the construction and testing of the cable is based in the context of electrical inspections. It is specified in the form of two voltage values: U0/U. Many ÖLFLEX® connection and control cables have a rated voltage of U0/U 300/500 V. • U0 is the rms-value (root mean square) of the voltage between a live conductor (core) and the earth (ground). The earth can be a metallic cable sheath (copper braid) or an earthed surrounding medium such as the metal casing of a device or control cabinet. The U0 value is lower than the U value, as there is as electrical separation only one layer of insulation between the live copper conductor and surrounding metallic medium. • U is the rms-value of the voltage between 2 live conductors (cores) of a multicore cable or within a system of single cores. The U value is higher than the U0 value since there are always as electrical separation two layers of insulation between two live copper conductors in a multi-core cable or between two single cores in a switch cabinet. The electrical voltage is measured in Volt (V).

author: themadhippy
author_rank: Super Contributor
timestamp: March 11, 2023, 12:48:07 am
content: Quotesteel wire armourcy cableMicc(pyro)just a couple of examples

author: robsims
author_rank: Frequent Contributor
timestamp: March 11, 2023, 01:16:20 am
content: Quote from: themadhippy on March 11, 2023, 12:48:07 amNice!!!, so as i understand what is written on a website, U0 is the max rms voltage between a live wire and the metal sheet of a cable or the max rms voltage between a live wire and the metal body (earth) of a machine when the insulation of the live wire comes in contact with the metal body. So the thinner the insulation the lower the voltage. U is the max voltage between two live wires and is higher. This is because the insulation between two live wires is thicker so the voltage can be higher. That's why U is always higher than U0. Very logical explanation. So it all comes down to insulation. It has nothing to do with single phase or three phase.