Both sides remain in qualifying places with Essex top on run-rate and Glamorgan in third.
Essex make one change with Jamie Porter replacing Matt Quinn in their seam attack., while Glamorgan are unchanged.
Glamorgan won the toss and captain Jacques Rudolph chose to bat first on a cloudy afternoon.
Both teams face a hectic schedule of five limited-overs games in a week, as the One-Day Cup and T20 Blast groups both reach their climax.
Essex lost to Surrey by 66 runs despite 131 from Jesse Ryder, while Glamorgan went down by 33 runs away to Somerset,  in their previous matches.
"Though we're still in the top four, a win (over Somerset) would have set us on the way to a quarter-final- we want to win every game" Glamorgan fast bowler Michael Hogan told BBC Wales.
Essex coach Chris Silverwood told the club's website "The players know they have to bring consistency to their game,  if we are to ensure Essex are in the quarter-finals of both white-ball competitions, and we need to start with a win against Glamorgan."
Glamorgan won the 2015 match in Cardiff by 146 runs, with Colin Ingram hitting 130.
Essex : Browne, Westley, Ryder, Bopara (c), Lawrence, ten Doeschate, Foster, Zaidi, Napier, Masters, Porter.
Glamorgan: Lloyd, Rudolph (c), Bragg, Ingram, Donald, Wallace, Wagg, Meschede, Salter, van der Gugten, Hogan.

It was responding to a global study that suggested investing heavily in computers and classroom technology does not improve pupils' performance.
Highland Council has a plan to give every P6 to S6 students access to a tablet computer.
The local authority said the study also suggested that good teaching coupled with technology could benefit pupils.
The study of schoolchildren in more than 70 countries was carried out for the Organisation for Economic Co-operation and Development (OECD).
The think tank said frequent use of computers in schools was more likely to be associated with lower results.
The OECD's education director Andreas Schleicher said school technology had raised "too many false hopes".
But he added that the findings of the report should not be used as an "excuse" not to use technology, but as a spur to finding a more effective approach.
A spokesperson for Highland Council said: "Highland Council is committed to providing technology rich environments in schools, this includes appropriate access to technology for learning purposes.
"We are also committed to reducing inequalities where pupils do not feel excluded due to not having access to appropriate technology for their learning.
"We welcome the OECD report which demonstrates that when good teaching is coupled with appropriate technology it leads to improved outcomes for children and young people."
Highland Council plans to increase the availability of the devices for P1 to 5 pupils to share and give every P6 to S6 student their own tablet.
The proposal is also expected to help the council save money.
The project could form part of the local authority's new IT contract.
The aspiration was first discussed at a meeting of Highland Council's education, children and adult services committee in February.
Councillors heard that the use of tablets at Alness Academy and other schools had led to pupils producing better work.

John Fell, from Cumbria, is confident his 1.5kg ostrich egg covered in meat will earn him a Guinness World Record.
The existing record was set in 2008 in a London hotel when the snack produced weighed in at just over 6kg.
Mr Fell, Calderbridge, used 10kg of sausage meat and 1.5kg of breadcrumbs - about five loaves' worth.
It took him seven hours to cook his giant snack - the Ostrich egg was boiled for two hours and it took another three hours to cook the meat.
His attempt in a chip shop in Leith, Edinburgh, was filmed by Scottish television and watched by an official record monitor, with each stage carefully documented.
Mr Fell said: "I am a judge in a competition entitled the Egg Awards 2014 - we support local egg producers, and encourage people not to buy supermarket eggs.
"This is to promote local eggs. But ironically the egg used had to come from Mexico as I couldn't track any down in England.
"About 60 people watched me cook the egg and helped us eat it - it was nice - the meat was based on an old Cumberland sausage recipe."
He now has to wait six weeks to find out whether his enormous savoury delicacy will be officially classed as a record-breaker.
Essex and Glamorgan will be looking to recover from defeats as they meet at Chelmsford in the One-Day Cup.
[NEXT_CONCEPT]
Highland Council has said it remains committed to giving more schoolchildren access to technology.
[NEXT_CONCEPT]
A chef is hoping he has cooked the world's biggest scotch egg - a whopping 11kg (24lbs) - which took three hours to deep fry.